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 



