Scientific American Supplement, No. 384, May 12, 1883

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Faraday's great discovery of magneto-induction was next noticed, and the
original instrument by which he had elicited the first electric spark
before the members of the Royal Institution in 1831, was shown in
operation. It was proved that although the individual current produced
by magnetoinduction was exceedingly small and momentary in action, it
was capable of unlimited multiplication by mechanical arrangements of a
simple kind, and that by such multiplication the powerful effects of the
dynamo machine of the present day were built up. One of the means for
accomplishing such multiplication was the Siemens armature of 1856.
Another step of importance was that involved in the Pacinotti ring,
known in its practical application as the machine of Gramme. A third
step, that of the self exciting principle, was first communicated by Dr.
Werner Siemens to the Berlin Academy, on the 17th of January, 1867, and
by the lecturer to the Royal Society, on the 4th of the following
month. This was read on the 14th of February, when the late Sir Charles
Wheatstone also brought forward a paper embodying the same principle.
The lecturer's machine, which was then exhibited, and which might be
looked upon as the first of its kind, was shown in operation; it had
done useful work for many years as a means of exciting steel magnets.
A suggestion contained in Sir Charles Wheatstone's paper, that "a very
remarkable increase of all the effects, accompanied by a diminution in
the resistance of the machine, is observed when a cross wire is placed
so as to divert a great portion of the current from the electro-magnet,"
had led the lecturer to an investigation read before the Royal Society
on the 4th of March, 1880, in which it was shown that by augmenting the
resistance upon the electro-magnets 100 fold, valuable effects could be
realized, as illustrated graphically by means of a diagram. The most
important of these results consisted in this, that the electromotive
force produced in a "shunt-wound machine," as it was called, increased
with the external resistance, whereby the great fluctuations formerly
inseparable from electric arc lighting could be obviated, and thus,
by the double means of exciting the electro-magnets, still greater
uniformity of current was attainable.

The conditions upon which the working of a well conceived dynamo machine
must depend were next alluded to, and it was demonstrated that when
losses by unnecessary wire resistance, by Foucault currents, and by
induced currents in the rotating armature were avoided, as much as 90
per cent., or even more, of the power communicated to the machine was
realized in the form of electric energy, and that _vice versa_ the
reconversion of electric into mechanical energy could be accomplished
with similarly small loss. Thus, by means of two machines at a moderate
distance apart, nearly 80 per cent, of the power imparted to one machine
could be again yielded in the mechanical form by the second, leaving
out of consideration frictional losses, which latter need not be
great, considering that a dynamo machine had only one moving part
well balanced, and was acted upon along its entire circumference by
propelling force. Jacobi had proved, many years ago, that the maximum
efficiency of a magneto-electric engine was obtained when

e / E = w / W = 1/2

which law had been frequently construed, by Verdet (Theorie Mecanique
de la Chaleur) and others, to mean that one-half was the maximum
theoretical efficiency obtainable in electric transmission of power, and
that one half of the current must be necessarily wasted or turned into
heat. The lecturer could never be reconciled to a law necessitating such
a waste of energy, and had maintained, without disputing the accuracy of
Jacobi's law, that it had reference really to the condition of maximum
work accomplished with a given machine, whereas its efficiency must be
governed by the equation:

e / E = w / W = nearly 1

From this it followed that the maximum yield was obtained when two
dynamo machines (of similar construction) rotated nearly at the same
speed, but that under these conditions the amount of force transmitted
was a minimum. Practically the best condition of working consisted in
giving to the primary machine such proportions as to produce a current
of the same magnitude, but of 50 per cent, greater electromotive force
than the secondary; by adopting such an arrangement, as much as 50 per
cent, of the power imparted to the primary could be practically received
from the secondary machine at a distance of several miles. Professor
Silvanus Thompson, in his recent Cantor Lectures, had shown an ingenious
graphical method of proving these important fundamental laws.

The possibility of transmitting power electrically was so obvious that
suggestions to that effect had been frequently made since the days of
Volta, by Ritchie, Jacobi, Henry, Page, Hjorth, and others; but it
was only in recent years that such transmission had been rendered
practically feasible.

Just six years ago, when delivering his presidential address to the Iron
and Steel Institute, the lecturer had ventured to suggest that "time
will probably reveal to us effectual means of carrying power to great
distances, but I cannot refrain from alluding to one which is, in my
opinion, worthy of consideration, namely, the electrical conductor.
Suppose water power to be employed to give motion to a dynamo-electrical
machine, a very powerful electrical current will be the result, which
may be carried to a great distance, through a large metallic conductor,
and then be made to impart motion to electromagnetic engines, to ignite
the carbon points of electric lamps, or to effect the separation of
metals from their combinations. A copper rod 3 in. in diameter would
be capable of transmitting 1,000 horse power a distance of say thirty
miles, an amount sufficient to supply one-quarter of a million candle
power, which would suffice to illuminate a moderately-sized town." This
suggestion had been much criticised at the time, when it was still
thought that electricity was incapable of being massed so as to deal
with many horse power of effect, and the size of conductor he had
proposed was also considered wholly inadequate. It would be interesting
to test this early calculation by recent experience. Mr. Marcel Deprez
had, it was well known, lately succeeded in transmitting as much as
three horse power to a distance of 40 kilometers (25 miles) through
a pair of ordinary telegraph wires of 4 millimeters in diameter. The
results so obtained had been carefully noted by Mr. Tresca, and had been
communicated a fortnight ago to the French Academy of Sciences. Taking
the relative conductivity of iron wire employed by Deprez, and the 3
in. rod proposed by the lecturer, the amount of power that could be
transmitted through the latter would be about 4,000 horse power. But
Deprez had employed a motor-dynamo of 2,000 volts, and was contented
with a yield of 32 per cent. only of the energy imparted to the primary
machine, whereas he had calculated at the time upon an electromotive
force of 200 volts, and upon a return of at least 40 per cent. of the
energy imparted. In March, 1878, when delivering one of the Science
Lectures at Glasgow, he said that a 2 in. rod could be made to
accomplish the object proposed, because he had by that time conceived
the possibility of employing a current of at least 500 volts. Sir
William Thomson had at once accepted these views, and with the
conceptive ingenuity peculiar to himself, had gone far beyond him, in
showing before the Parliamentary Electric Light Committee of 1879, that
through a copper wire of only 1/2 in. diameter, 21,000 horse power might
be conveyed to a distance of 300 miles with a current of an intensity
of 80,000 volts. The time might come when such a current could be dealt
with, having a striking distance of about 12 ft. in air, but then,
probably, a very practical law enunciated by Sir William Thomson would
be infringed. This was to the effect that electricity was conveyed at
the cheapest rate through a conductor, the cost of which was such
that the annual interest upon the money expended equaled the annual
expenditure for lost effect in the conductor in producing the power to
be conveyed. It appeared that Mr. Deprez had not followed this law in
making his recent installations.

Sir William Armstrong was probably first to take practical, advantage of
these suggestions in lighting his house at Cragside during night time,
and working his lathe and saw bench during the day, by power transmitted
through a wire from a waterfall nearly a mile distant from his mansion.
The lecturer had also accomplished the several objects of pumping water,
cutting wood, hay, and swedes, of lighting his house, and of carrying on
experiments in electro-horticulture from a common center of steam power.
The results had been most satisfactory; the whole of the management
had been in the hands of a gardener and of laborers, who were without
previous knowledge of electricity, and the only repairs that had been
found necessary were one renewal of the commutators and an occasional
change of metallic contact brushes.

An interesting application of electric transmission to cranes, by Dr.
Hopkinson, was shown in operation.

Among the numerous other applications of the electrical transmission
of power, that to electrical railways, first exhibited by Dr. Werner
Siemens, at the Berlin Exhibition of 1879, had created more than
ordinary public attention. In it the current produced by the dynamo
machine, fixed at a convenient station and driven by a steam engine
or other motor, was conveyed to a dynamo placed upon the moving car,
through a central rail supported upon insulating blocks of wood, the two
working rails serving to convey the return current. The line was 900
yards long, of 2 ft gauge, and the moving car served its purpose of
carrying twenty visitors through the exhibition each trip. The success
of this experiment soon led to the laying of the Lichterfelde line, in
which both rails were placed upon insulating sleepers, so that the one
served for the conveyance of the current from the power station to the
moving car, and the other for completing the return circuit. This line
had a gauge of 3 ft. 3 in., was 2,500 yards in length, and was worked
by two dynamo machines, developing an aggregate current of 9,000 watts,
equal to 12 horse power. It had now been in constant operation since May
16, 1881, and had never failed in accomplishing its daily traffic.
A line half a kilometer in length, but of 4 ft. 81/2 in. gauge was
established by the lecturer at Paris in connection with the Electric
Exhibition of 1881. In this case, two suspended conductors in the form
of hollow tubes with a longitudinal slit were adopted, the contact being
made by metallic bolts drawn through these slit tubes, and connected
with the dynamo machine on the moving car by copper ropes passing
through the roof. On this line 95,000 passengers were conveyed within
the short period of seven weeks.

An electric tramway, six miles in length, had just been completed,
connecting Portrush with Bush Mills, in the north of Ireland, in the
installation of which the lecturer was aided by Mr. Traill, as engineer
of the company by Mr. Alexander Siemens, and by Dr. E. Hopkinson,
representing his firm. In this instance the two rails, 3 ft. apart, were
not insulated from the ground, but were joined electrically by means of
copper staples and formed the return circuit, the current being conveyed
to the car through a T iron placed upon short standards, and insulated
by means of insulate caps. For the present the power was produced by
a steam engine at Portrush, giving motion to a shunt-wound dynamo of
15,000 watts=20 horse power, but arrangements were in progress to
utilize a waterfall of ample power near Bush Mills, by means of three
turbines of 40 horse power each, now in course of erection. The working
speed of this line was restricted by the Board of Trade to ten miles an
hour, which was readily obtained, although the gradients of the line
were decidedly unfavorable, including an incline of two miles in length
at a gradient of 1 in 38. It was intended to extend the line six miles
beyond Bush Mills, in order to join it at Dervock station with the north
of Ireland narrow gauge railway system.

The electric system of propulsion was, in the lecturer's opinion,
sufficiently advanced to assure practical success under suitable
circumstances--such as for suburban tramways, elevated lines, and
above all lines through tunnels; such as the Metropolitan and District
Railways. The advantages were that the weight, of the engine, so
destructive of power and of the plant itself in starting and stopping,
would be saved, and that perfect immunity from products of combustion
would be insured The experience at Lichterfelde, at Paris, and another
electric line of 765 yards in length, and 2 ft. 2 in. gauge, worked
in connection with the Zaukerode Colliery since October, 1882, were
extremely favorable to this mode of propulsion. The lecturer however
did not advocate its prospective application in competition with the
locomotive engine for main lines of railway. For tramways within
populous districts, the insulated conductor involved a serious
difficulty. It would be more advantageous under these circumstances to
resort to secondary batteries, forming a store of electrical energy
carried under the seats of the car itself, and working a dynamo machine
connected with the moving wheels by means of belts and chains.

The secondary battery was the only available means of propelling vessels
by electrical power, and considering that these batteries might be made
to serve the purpose of keel ballast, their weight, which was still
considerable, would not be objectionable. The secondary battery was not
an entirely new conception. The hydrogen gas battery suggested by Sir
Wm. Grove in 1841, and which was shown in operation, realized in the
most perfect manner the conception of storage, only that the power
obtained from it was exceedingly slight. The lecturer, in working upon
Sir Wm. Grove's conception, had twenty-five years ago constructed
a battery of considerable power in substituting porous carbon for
platinum, impregnating the same with a precipitate of lead peroxidized
by a charging current. At that time little practical importance attached
however to the object, and even when Plante, in 1860, produced his
secondary battery, composed of lead plates peroxidized by a charging
current, little more than scientific curiosity was excited. It was
only since the dynamo machine had become an accomplished fact that
the importance of this mode of storing energy had become of practical
importance, and great credit was due to Faure, to Sellon, and to
Volckmar for putting this valuable addition to practical science into
available forms. A question of great interest in connection with the
secondary battery had reference to its permanence. A fear had been
expressed by many that local action would soon destroy the fabric of
which it was composed, and that the active surfaces would become coated
with sulphate of lead, preventing further action. It had, however,
lately been proved in a paper read by Dr. Frankland before the Royal
Society, corroborated by simultaneous investigations by Dr. Gladstone
and Mr. Tribe, that the action of the secondary battery depended
essentially upon the alternative composition and decomposition of
sulphate of lead, which was therefore not an enemy, but the best friend
to its continued action.

In conclusion, the lecturer referred to electric nomenclature, and to
the means for measuring and recording the passage of electric energy.
When he addressed the British Association at Southampton, he had
ventured to suggest two electrical units additional to those established
at the Electrical Congress in 1881, viz.: the watt and the joule,
in order to complete the chain of units connecting electrical with
mechanical energy and with the unit quantity of heat. He was glad to
find that this suggestion had met with a favorable reception, especially
that of the watt, which was convenient for expressing in an intelligible
manner the effective power of a dynamo machine, and for giving a precise
idea of the number of lights or effective power to be realized by its
current, as well as of the engine power necessary to drive it; 746 watts
represented 1 horse-power.

Finally, the watt meter, an instrument recently developed by his firm,
was shown in operation. This consisted simply of a coil of thick
conductor suspended by a torsion wire, and opposed laterally to a fixed
coil of wire of high resistance. The current to be measured flowed
through both coils in parallel circuit, the one representing its
quantity expressible in amperes, and the other its potential expressible
in volts. Their joint attractive action expressed therefore volt-amperes
or watts, which were read off upon a scale of equal divisions.

The lecture was illustrated by experiments, and by numerous diagrams and
tables of results. Measuring instruments by Professors Ayrton and Perry,
by Mr. Edison and by Mr. Boys, were also exhibited.

* * * * *

ON THE PREPARATION OF GELATINE PLATES.

[Footnote: Being an abstract of the introductory lecture to a course on
photography at the Polytechnic Institute, November 11.]

By E. HOWARD FARMER, F.C.S.

Since the first announcement of these lectures, our Secretary has asked
me to give a free introductory lecture, so that all who are interested
in the subject may come and gather a better idea as to them than they
can possibly do by simply leading a prospectus. This evening, therefore,
I propose to give first a typical lecture of the course, and secondly,
at its conclusion, to say a few words as to our principal object. As the
subject for this evening's lecture I have chosen, "The Preparation of
Gelatine Plates," as it is probably one of very general interest to
photographers.

Before preparing our emulsion, we must first decide upon the particular
materials we are going to use, and of these the first requisite is
nitrate of silver. Nitrate of silver is supplied by chemists in three
principal conditions:

1. The ordinary crystallized salt, prepared by dissolving silver in
nitric acid, and evaporating the solution until the salt crystallizes
out. This sample usually presents the appearance of imperfect crystals,
having a faint yellowish tinge, and a strong odor of nitrous fumes, and
contains, as might be expected, a considerable amount of free acid.

2. Fused nitrate, or "lunar caustic," prepared by fusing the
crystallized salt and casting it into sticks. Lunar caustic is usually
alkaline to test paper.

3. Recrystallized silver nitrate, prepared by redissolving the ordinary
salt in distilled water, and again evaporating to the crystallizing
point. By this means the impurities and free acid are removed.

I have a specimen of this on the table, and it consists, as you observe,
of fine crystals which are perfectly colorless and transparent; it is
also perfectly neutral to test paper. No doubt either of these samples
can be used with success in preparing emulsions, but to those who are
inexperienced, I recommend that the recrystallized salt be employed. We
make, then, a solution of recrystallized silver nitrate in distilled
water, containing in every 12 ounces of solution 11/4 ounces of the salt.

The next material we require is a soluble bromide. I have here specimens
of various bromides which can be employed, such as ammonium, potassium,
barium, and zinc bromides; as a rule, however, either the ammonium or
potassium salt is used, and I should like to say a few words respecting
the relative efficiency of these two salts.

1. As to ammonium bromide. This substance is a highly unstable salt.
A sample of ammonium bromide which is perfectly neutral when first
prepared will, on keeping, be found to become decidedly acid in
character. Moreover, during this decomposition, the percentage of
bromine does not remain constant; as a rule, it will be found to contain
more than the theoretical amount of bromine. Finally, all ammonium salts
have a most destructive action on gelatine; if gelatine, which has
been boiled for a short time with either ammonium bromide or ammonium
nitrate, be added to an emulsion, it will be found to produce pink
fog--and probably frilling--on plates prepared with the emulsion. For
these reasons, I venture to say that ammonium bromide, which figures so
largely in formulae for gelatine emulsions, is one of the worst bromides
that can be employed for that purpose, and is, indeed, a frequent source
of pink fog and frilling.

2. As to potassium bromide. This is a perfectly stable substance, can be
readily obtained pure, and is constant in composition; neither has it
(nor the nitrate) any appreciable destructive action on gelatine. We
prepare, then, a solution of potassium bromide in water containing in
every 12 ounces of solution 1 ounce of the salt. On testing it with
litmus paper, the solution may be either slightly alkaline or neutral;
in either case, it should be faintly acidified with hydrochloric acid.

The last material we require is the gelatine, one of the most important,
and at the same time the most difficult substance to obtain of good
quality. I have various samples here--notably Nelson's No. 1 and "X
opaque;" Coignet's gold medal; Heinrich's; the Autotype Company's; and
Russian isinglass.

The only method I know of securing a uniform quality of gelatine is to
purchase several small samples, make a trial emulsion with each, and buy
a stock of the sample which gives the best results. To those who do not
care to go to this trouble, equal quantities of Nelson's No. 1 and
X opaque, as recommended by Captain Abney, can be employed. Having
selected the gelatine, 11/4 ounces should be allowed to soak in water, and
then melted, when it will be found to have a bulk of about 6 ounces.

In order to prepare our emulsion, I take equal bulks of the silver
nitrate and potassium bromide solutions in beakers, and place them in
the water bath to get hot. I also take an equal bulk of hot water in a
large beaker, and add to it one-half an ounce of the gelatine solution
to every 12 ounces of water. Having raised all these to about 180 deg. F., I
add (as you observe) to the large beaker containing the dilute gelatine
a little of the bromide, then, through a funnel having a fine orifice,
a little of the silver, swirling the liquid round during the operation;
then again some bromide and silver, and so on until all is added.

When this is completed, a little of the emulsion is poured on a glass
plate, and examined by transmitted light; if the mixing be efficient,
the light will appear--as it does here--of an orange or orange red
color.

It will be observed that we keep the bromide in excess while mixing. I
must not forget to mention that to those experienced in mixing, by
far the best method is that described by Captain Abney in his Cantor
lectures, of keeping the silver in excess.

The emulsion, being properly mixed, has now to be placed in the water
bath, and kept at the boiling point for forty-five minutes. As,
obviously, I cannot keep you waiting while this is done, I propose to
divide our emulsion into two portions, allowing one portion to stew, and
to proceed with the next operation with the remainder.

Supposing, then, this emulsion has been boiled, it is placed in cold
water to cool. While it is cooling, let us consider for a moment what
takes place during the boiling. It is found that during this time the
emulsion undergoes two remarkable changes:

1. The molecules of silver bromide gradually aggregate together, forming
larger and larger particles.

2. The emulsion increases rapidly in sensitiveness. Now what is the
cause, in the first place, of this aggregation of molecules: and, in the
second place, of the increase of sensitiveness? We know that the two
invariably go together, so that we are right in concluding that the same
cause produces both.

It might be thought that heat is the cause, but the same changes take
place more slowly in the cold, so we can only say that heat accelerates
the action, and hence must conclude that the prime cause is one of the
materials in the emulsion itself.

Now, besides the silver bromide, we have in the emulsion water,
gelatine, potassium nitrate, and a small excess of potassium bromide;
and in order to find which of these is the cause, we must make different
emulsions, omitting in succession each of these materials. Suppose we
take an emulsion which has just been mixed, and, instead of boiling
it, we precipitate the gelatine and silver bromide with alcohol; on
redissolving the pellicle in the same quantity of water, we have an
emulsion the same as previously, with the exception that the niter and
excess of potassium bromide are absent. If such an emulsion be boiled,
we shall find the remarkable fact that, however long it be boiled, the
silver bromide undergoes no change, neither does the emulsion become
any more sensitive. We therefore conclude, that either the niter or the
small excess of potassium bromide, or both together, produce the change.

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