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°, 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.
* * * * *