Scientific American Supplement, No. 433, April 19, 1884

V >> Various >> Scientific American Supplement, No. 433, April 19, 1884

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NEW INSTRUMENTS FOR MEASURING ELECTRIC CURRENTS AND ELECTRO-MOTIVE
FORCE.

By Messrs. R. E. CROMPTON and GISBERT KAPP.

[Footnote: Paper read before the Society of Telegraph Engineers, 14th
February, 1884.]

In consequence of the rapid development of that part of electrical
science which may be termed "heavy electrical engineering," reliable
measuring instruments specially suitable for the large currents
employed in lighting and transmission of energy have become an absolute
necessity. As usual, demand has stimulated supply, and many ingenious
and useful instruments have been invented, the manufacture of which
forms at the present day an important industry. Mr. Shoolbred, in a
paper which he recently read before this Society, gave a full and
interesting account of the labors of our predecessors in this field.
To-day we add to the list then given a class of instruments invented by
us, examples of which are now before you on the table. We have preferred
to call them current and potential indicators in preference to meters,
considering that the latter term, or rather termination, ought to be
applied rather to integrating instruments, which the necessities of
electric lighting, we believe, will soon bring into extensive use. The
principal aim in the design of these indicators has been to obtain
instruments which will not alter their calibration in consequence of
external disturbing forces. If this object can be attained, then it will
be possible to divide the scale of each instrument directly into amperes
or volts, as the cause may be, and thus avoid the use of a coefficient
of calibration by which the deflection has to be multiplied. This is an
important consideration when it is remembered that in many cases
these instruments have to be used by unskilled workmen, to whom a
multiplication involving the use of demical fractions is a tedious and
in some cases even an impossible task.

[Illustration: FIG. 1. FIG. 2.]

All measurements are comparative. We measure weights or forces by
comparison with some generally known and accepted unit standard
weights, lengths, areas, and volumes, by comparison with a unit length,
resistance by a standard ohm, and so forth. In the same way currents
could be measured by comparison with a standard current: but this
would be a troublesome process, not only on account of the apparatus
necessary, but also because it would be a matter of some difficulty to
have a standard current always ready for use. In general, measurement
by direct comparison with a standard unit is discarded for the more
indirect method of measuring not the current itself, but its chemical,
mechanical, or magnetic effect. The chemical method is very accurate if
a proper density of current through the surface of the electrodes be
used,[1] but since it requires a considerable time, and, above all, an
absolutely constant current, its use is almost entirely restricted
to laboratory work and to the calibration of other instruments. For
practical ready use, instruments employing the mechanical or magnetic
effect of the current are alone suitable. We weigh, so to speak, the
current against the force of a magnet, of a spring, or of gravity.
The measurement will be exact if the thing against which we weigh or
counterbalance the current itself retains its original standard value.
Where permanent magnets or springs are used as a balancing force, this
condition of constancy in our weights and measures is not always fully
maintained, and to make matters worse, there is no visible sign by which
a change, should it have occurred, can be readily detected. A spring
may have been overstrained or a steel magnet may have become weakened
without showing the least alteration in outward appearance. To overcome
this difficulty, the obvious remedy is not to use springs or steel
magnets at all, but to substitute for these some other force which
should be either absolutely constant, such as the force of gravity, or
at least should, vary only within narrow limits, and this in accordance
with a definite law. This latter condition can be fulfilled by the
employment of electro-magnets.

[Footnote 1: According to recent experiments made by Dr. Hammerl, the
density of current in a copper voltameter should be half an ampere per
square inch of surface.]

[Illustration: FIG 3.]

To imitate with an electro magnet as nearly as possible a permanent
magnet, so that the former can be used to replace the latter, it is
necessary that the magnetism in the iron core should remain constant.
This could, of course, be done by exciting the electro magnet with a
constant current from a separate source. (In a recent note to the Paris
Academy of Science, M.E. Ducretet described a galvanometer with steel
magnet, which is surrounded by an exciting coil. When recalibration
appears necessary, a known standard current from large Daniell cells is
sent through this coil during a certain time, and thus the magnet is
brought back to its original degree of saturation. M. Ducretet also
mentions the use of a soft iron bar instead of a steel magnet, in which
case the current from the Daniell cells must be kept on during the time
an observation is taken.) But such a system would appear to be too
complicated for ready use. Moreover, some sort of indicator would be
required by which we could make sure that the exciting current has the
normal strength.

[Illustration: FIG 4.]

The plan we adopt is to excite the electro magnet by the whole or a part
of the current which is to be measured. Since this current varies, the
power exciting the core of the electro magnet must also vary; and since
we require the core to have as nearly as possible a permanent magnetic
force, we are brought face to face with the question, whether an electro
magnet can be constructed that has a constant moment under varying
exciting currents. This question has been answered by the well known
experiments of Jacobi, Dub, Mueller, Weber, and others. To get an
absolutely constant magnetic moment, is not possible, but between
certain limits we can get a very near approximation to constancy.

[Illustration]

The relation between exciting power and magnetic moment is very
complicated, depending not only on the dimensions and shape of the core
and the manner of winding, but also on the chemical constitution of the
iron of the core. It is not possible, or at least it has hitherto not
been found possible, to embody all these various elements into an exact
mathematical formula, which would give the magnetic moment as a function
of the exciting current; but the above mentioned experiments have shown
that within certain limits, and in the neighborhood of the point of
saturation, the relation between the two is that of an arc to its
geometrical tangent. It will be seen that for large angles the arc
increases much slower than the tangent; that is, for strongly excited
cores, a very large increase of the exciting current will produce only
a slight increase of magnetic moment. If Mueller's formula were correct
for all currents, absolute saturation could only be reached with an
infinite current. Whether this be the case or not, it is certain that
the greater the exciting current the less will a variation in it affect
the magnetic moment of the core. To imitate as nearly as possible
permanent steel magnets, it is therefore necessary to use electro
magnets, the cores of which are easily saturated. The core should be
thin and long and of the horseshoe type; the amount of wire wound round
it should be large in comparison with the size of the core.

[Illustration]

Here is a magnet partly wound which was used in one of our earliest
experiments, but which was a failure on account of having far too much
mass in the core in comparison with the amount of copper wire wound
round it. Since then we have greatly diminished the iron and increased
the copper. The cores of the instruments on the table are composed of
two or three No. 18 b.w.g. charcoal iron wires, and are wound with one
layer of 0'120 inch wire in the case of the current indicators, and
eighteen layers of 0.0139 inch wire in the case of the potential
indicator. If from the diagram, Fig. 1, we plot a curve the abscissae of
which represent exciting current, and the ordinates magnetic moment of
the soft iron core, we find that a considerable portion of the curve
is almost a straight and only slightly inclined line. If it, were a
horizontal straight line the core would be absolutely saturated, but
such as it is, it answers the purpose sufficiently well, for with a
variation of exciting current from 10 to 100 amperes the magnetic moment
varies but slightly. If a small soft iron or magnetic steel needle, _n
s_, be suspended between the poles, S N, of an electro magnet of such
proportions as described above, and the current, after exciting the
electro magnet, _e e_, be lead round the coils, DD, it will be found
that for all currents between 10 and 100 amperes the needle, _n
s_, shows a definite deflection for each current. Here we have a
galvanometer with permanent calibration. In this case the deflection of
the needle will not strictly follow the law of tangents, because the
directing power of the electro magnet is not absolutely constant; but
whatever the exact ratio between deflection and current may be, it must
always remain the same, and to each angle of deflection corresponds one
definite strength of current.

[Illustration]

The force with which the electro magnet tends to keep the needle in its
zero position, that is, in line with the poles, S N, is due partly to
the magnetism of the core, which is nearly constant, and partly to the
magnetic influence of the coils, _ee_, themselves, which is, of course,
simply proportional to the current. The total magnetic force acting on
the needle is, therefore, represented by the sum of these two forces,
and consequently not nearly so constant as might be desired in order to
get a good imitation of a tangent galvanometer with a permanent magnet.
In the diagram, Fig. 2, the curve, O A B, represents the magnetic moment
of the iron core, the straight line, ODE, that of the exciting coils per
se, and the dotted line, O F M, the sum of the two, obtained by adding
for every current, O C, the respective ordinates, CD and C A.

CF = CD + CA

The rise of this curve shows that the force which tends to bring the
needle back to its zero position increases with the current, though at a
slower ratio than the deflecting force of the current. It follows from
this that for large currents the increment in the angle of deflection is
comparatively small, and the divisions on the scale whereon the current
is to be read off would come too near together to allow accurate
readings to be taken. In other words, the range of accurate reading in
an instrument so constructed would only be limited. But it is very easy
to eliminate the magnetic effect of the coils of the electro magnet on
the needle, by introducing an opposite magnetic effect, so that only
that part of the force remains which belongs to the soft iron core
proper. One way of doing this is by surrounding the needle with a coil,
the plane of which is at right angles to the line, S N, and coupling
this coil in series with the deflecting coil, D D. If the proportions
of this transverse coil and the direction of the current through it
be properly chosen, its magnetic effect can be made to exactly
counterbalance that of the exciting coils, _e e_, without perceptibly
weakening the magnetism of the iron core. But instead of employing two
coils, one parallel and the other transversely to the zero position of
the needle, we can obtain the same result in a more simple manner with
one coil only, if this be placed at such an angle that its magnetic
effect can be substituted for the combined effects of the two coils. In
other words, we set the deflecting coil, D D, at a certain angle to the
zero position of the needle.

A similar arrangement, though not precisely for the same purpose, has
already been suggested and tried by Messrs. Deprez, Carpentier, Ayrton,
and Perry, in galvanometers with permanent steel magnets. If the coil,
D D, be so placed, the deflecting force which now acts obliquely can be
considered as the resultant of two forces, one acting at right angles to
the line, S N, as in an ordinary galvanometer, and the other parallel to
this line, but in a sense opposed to the action of the electro magnet
and its exciting coils. If the angle of obliquity be so chosen that this
latter component exactly equals the magnetic effect of the exciting
coils _per se_, an equality which holds good for all currents, then we
shall have an almost perfect imitation of a tangent galvanometer with
permanent magnets. But we can go a step further than this; we can
overbalance the exciting coils by setting the deflecting coil at a
greater angle than necessary for the mere elimination of the former, and
thus attain that an increase of current results in a slight weakening of
the field in which the needle swings, thus allowing the increment of the
angle of deflection to be comparatively large even for large currents.
In this way it is possible to obtain a more evenly divided scale than
in the case when the deflection follows the law of tangents, as in an
ordinary tangent galvanometer. This principle of overbalancing the
exciting coils is shown on diagram, Fig. 2. The straight line, O G,
represents the magnetic effect on the needle of that component of the
deflecting force which is parallel, but in sense opposed to S N;
as mentioned above, the magnetic effect of the exciting coils is
represented by the straight line, O E. The combined effect of these two
forces on the needle is represented by the line, O K, the ordinates
of which must be deducted from those of the curve, O A B, in order to
obtain the total directing force due to each current. This is shown by
the curve, O P Q, shown in a thick full line. This curve shows how
the directing force or strength of field in which the needle swings
decreases with an increasing current. That this does actually take place
can easily be proved by experiment.

Fig. 4 shows two curves; the one drawn in a full line is obtained
by plotting the deflection in degrees of the needle of a potential
indicator as abscissae, and the corresponding electromotive forces
measured simultaneously on a standard instrument as ordinates; the
dotted line shows what this curve would be with an ordinary tangent
galvanometer.

The needle of the potential indicator is mounted at the lower end of
a steel axle, to the upper end of which is fastened a light aluminum
pointer, whereby the deflection of the needle can be read off on a scale
divided directly into volts. The scale is placed within a circular dial
plate with glass cover, giving sufficient room for the pointer to swing
all round, and the needle is placed within a central tube fitting it
closely, which acts as a damper and so makes the instrument almost
dead beat. Tube and dial are in one casting. The electro magnet is of
horseshoe form fastened to a central tubular stand, which also serves
to support the two deflecting coils, one on either side of it. The tube
within which the magnetic needle swings is inserted into the stand,
which is bored out to the external diameter of the tube. The electro
magnet and deflecting coils are wound with from 50 to 100 ohms of fine
insulated copper wire, and an additional resistance coil of from 450
to 900 ohms of German silver is added, which can, however, be short
circuited by depressing a key when the instrument has to be used for
reading low electromotive forces. In this case the indication of the
pointer must be divided by ten. If a current be sent through the
instrument the wrong way, the needle turns through an angle of 180 deg., and
thus brings the pointer to the side of the dial opposite to where the
scale is. In this position no reading can be taken, and to facilitate
the sending of the current in the right direction a commutator is added,
and the same is so coupled up that when the pointer stands over the
scale the handle on the commutator points to the positive terminal
screw. There is a limit of electromotive force below which the indicator
fails to give reliable readings. For instance, an instrument wound with
100 ohms of copper wire and 900 ohms of German silver can be used for
electromotive forces varying between 300 and 3 volts, but would not be
reliable for measuring less than 3 volts.

For very exact measurements the instrument should be placed north and
south, in the same position in which it was calibrated. Two different
patterns of current indicators are on the table; one with double needles
suspended on a point in the way compass magnets are suspended, the other
with one lozenge shaped needle mounted on an axle and pivoted on jewels,
in every way similar to the needle of the potential indicator first
described.

For measurements of currents from 10 amperes upward, there is no need
to employ a complete coil as the deflecting agent; one half-coil or one
strip passing close under the needle gives sufficient deflecting force,
and thus the construction of the instrument is rendered extremely
simple. The current, after entering at one of the flat electrodes,
splits in two parts, each part passing round the winding of an electro
magnet of horseshoe form, the similar poles of both magnets pointing
toward each other and toward the needle. After traversing the winding,
the current unites again, and passes through a metal strip close under
the needle, and finally out of the instrument by the other electrode,
which lies close under that at which the current entered, but is
insulated from it by a sheet of fiber. The metal strip is set at an
angle, to balance or overbalance, as may be preferred, the magnetic
influence of the exciting coils. The effect of this overbalancing is
shown in Fig. 5, where the full curve represents the current as a
function of the deflection--obtained by comparison with a standard
instrument--and the dotted curve shows what that relation between
deflection and current would be if the law of tangents held good for
these instruments. It will be seen that, about the middle of the scale,
the dotted line coincides nearly with the full line, while at the
extreme end of the scale the dotted line is higher. From this follows,
that if we compare our indicator from which this curve was taken with
any form of tangent instrument showing an equal angle of deflection at
the medium reading, it will be seen that the needle of our indicator
will be deflected to a greater angle at high readings than that of the
tangent galvanometer. Consequently, the divisions on the scale will
be widest apart in our instruments, which greatly facilitates high
readings.

* * * * *

SECONDARY BATTERIES.

The Consolidated Electric Light Company has now completed the secondary
battery which has for some time engaged the attention of its officers,
and their regular manufacture and use for electric lighting stations
have been fairly entered upon. Among other places to which the batteries
have been sent and put into work is Colchester, where the company
has for some time had an installation at work, chiefly employing
incandescent lamps. The battery consists of lead electrodes, anode and
cathode being of the same character. They are constructed of narrow
ribbons of lead, each element being made from long lengths of the ribbon
about or nearly 0.20 in. width, rolled together into a flat cake like
rolls of narrow webbing, as illustrated by the annexed diagram, Fig.
1, the greater part of the ribbon being very thin and flat; but
intermediate thicker ribbons are also employed, as in Fig. 2, this
thicker ribbon being corrugated as shown, and affording passage room
for the circulation of the electrolyte. From four to eight coils of the
plain ribbons are between every pair of corrugated ribbons. They are
wound up together tightly, and pressed into the nearly rectangular form
shown. The bar for suspending the coil plates so made in the cells is
soldered to the coil. The object of this construction is of course to
obtain large lead surface, and of course a much larger surface is so
obtained than could be practically obtained from plain lead plates in
the same compass. A battery thus made may be seen at the offices of the
company, 110 Cannon Street.

[Illustration: FIG. 1. FIG. 2.]

A very ingenious device for cutting the battery out of circuit when
charged as much as is thought desirable is used by the company. In a
cell is an element which has a determined lower capacity than those in
the rest of the battery. Over this element is placed a gas-tight chamber
in which is a diaphgram, this diaphragm being of very flexible material
placed in the cover of the box of cells. When charging has proceeded as
long as is desirable, or proceeds too fast, hydrogen is evolved, and
this collecting in the chamber referred to acts upon the diaphragm, and
by means of a rod connected thereto, switches the current, which is
supplied to an electro-magnet and by which circuit is made through the
medium of mercury contacts. The object, of this is to save the
battery from destruction by over-charging or charging by too large a
current.--_The Engineer_.

* * * * *

ACETYLENE FROM IODOFORM.

P. CAZENEUVE publishes in the _Comptes Rendus_ a new method for the
preparation of acetylene, which consists in mixing iodoform intimately
with moist and finely divided silver. An abundant evolution of acetylene
takes place without heating. The reaction is represented by the
following formula: 2CHI_{3} + 6Ag = C_{2}H_{2} + 6 AgI. The
decomposition of the iodoform is hastened if the silver is mixed with
finely divided copper, such as can be obtained by precipitating it from
its sulphate by means of zinc.

Cazeneuve also observed that most metals which have any affinity for
iodine will decompose iodoform in the presence of water, forming
acetylene and an iodide of the metal. By the use of zinc he obtained a
liquid having a pleasant ethereal odor, and a gas mixture that contained
besides acetylene an iodine compound which burned with a purple-edged,
fawn-colored flame.

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WHEN DOES AN ELECTRICAL SHOCK BECOME FATAL?

In this age of electricity and electric wires carrying currents of
various intensity, the question of danger arising from contact with them
has caused considerable discussion. An examination into the facts as
they exist may therefore enlighten some who are at present in the dark.

To begin with, we often hear the question asked--why is it that certain
wires carrying very large currents give very little shock, whereas
others, with very small currents, may prove fatal to those coming in
contact with them? The answer to this is--that the shock a person
experiences does not depend upon the current _flowing in the wires_, but
upon the current _diverted from them_ and _flowing through the body_.

The muscular contraction due to a galvanic current, which was first
observed in the frog, gives a good illustration of the fact that it
requires only a very minute current to flow through the muscles in order
to contract them. Thus the simple contact of pieces of zinc and copper
with the nerves generated current sufficient to excite the muscles--a
current which would require a delicate galvanometer for its detection.
What is true of the muscles of the frog holds good also for the human
muscles; they too are very susceptible to the passage of a current.

In order to determine the current which proves fatal we need only to
apply the formula which expresses Ohm's law, viz., C=E/R, or the current
(ampere) equals the electromotive force (volt) divided by the resistance
(ohm).

According to the committee of Parliament investigation, the
electromotive force dangerous to life is about 300 volts; this then is
the quantity, E, in the formula. There remains now only to determine the
resistance in ohms which the body offers to the passage of the current.
In order to obtain this, a series of measurements under different
conditions were made. On account of the nature of the experiment a high
resistance Thomson reflecting galvanometer was used, with the following
results.

When the hands had been wiped perfectly dry, the resistance of the body
was about 30,000 ohms; with the hands perspiring ordinarily it fell to
10,000 ohms; whereas when they were dripping wet it was as low as 7,000
ohms. Our readers can judge this resistance best when we state that the
Atlantic cable offers a resistance of 8,000 ohms.

Taking an ordinary condition of the body, with the hands perspiring as
usual, we would have the resistance equal to 10,000 ohms. Applying the
two known quantities in the formula, we get:

C = (300 / 10,000) - (1 / 33.333+)

This means, therefore, that when the electromotive force or potential is
great enough to send a current of 1/33 ampere through the body, fatal
results will ensue. This current is so minute that it would deposit only
about 6 _grains_ of copper in _one hour_, a grain being 1/7,000 of a
pound.

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