950
��Popular Science Monthly
��the increase in length causing the pulley to rotate, carrying the arm with it. This arm movement causes the silk fiber to rotate the shaft, which carries the needle. Hot-wire meters deflect in the same direction for currents in either direction and are equally accurate on direct and alternating current. They are not af- fected by stray magnetism and have the advantage that they can be calibrated on direct for use on alternating current. This type of instrument is not generally employed in practical work, but is valua-
��Uovoble needle of meter "••...
���Non-OAidizoble Hefal wire
��ipring odjus*cr
��.Shaft with pulley
Hot-wire meters deflect in the same di- rection for currents in either direction
��ble for measuring currents of high fre- quency, such as are used in wireless telegraphy.
The simplest form of meter employing the electromagnetic effect is the gal- vanometer, an instrument for detecting small currents. In Fig. 3 is a so-called tangent galvanometer, the operating prin- ciple of which is as follows: If a coil of wire is placed in the plane of the magnetic meridian {N and S line), and a magnetic needle is suspended at its center, a current passing through the coil will deflect the needle away from the magnetic meridian by an angle whose tangent is proportional to the current strength.
The D'Arsonval galvanometer is a modification of the tangent galvanometer, with reversed positions of moving and fixed elements. In this instrument, there is a small solenoid oscillating under the directive force of a permanent magnet,
��Composs
���The simplest form of a tangent galvanometer
��instead of a suspended magnet moving under the directive force of a coil. Cur- rent which is lead to the coil through its suspension causes it to rotate about its
axis, with a ten- -^^^^^^
dency to place itself at right angles to the lines of force. A pointer may be fastened to the coil to show the deflections, or a mirror may be used, from which a re- flected light ray forms the point- er. In most forms of this instrument (see Fig. 4) a soft iron core is supported be- tween the magnet poles from the rear, leaving a space between core and magnet, in which the coil swings. This tends to increase the magnetic field in which the coil rotates, giving a more sensitive deflection.
Galvanometers are generally used to indicate the presence and direction of electric currents rather than to ascertain their intensity. The D'Arsonval princi- ple, however, can be applied to portable meters for the direct measurement of
voltage and ^^^ Toterm.
amperage. When used to measure po- tential or E. M. F. they are called voltme- ters, and are graduated to read in volts; and when measur- ing current, they are called am- meters, and read the cur- rent in am- peres. In Fig. 5 is shown
��Mirror
���To term.
��A soft iron core sup- ported between per- manent magnetic poles
��the D'Arsonval principle as applied to in- struments of this sort.
In fundamental principles, am.:ieters and voltmeters are alike, inasmuch as the deflecting torque of the instrument is
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