Popular Science MontJily 



05 1 



proportional to the strength of the mag- 

 netic field multiplied by the current 

 flowing in the movable coil. An ammeter, 

 since it measures the current flowing in 

 the conductor, must be placed in series in 

 the circuit and hence its coil should have 

 as low resistance as possible. On the 

 other hand, 



a voltmeter, 

 inasmuch as 

 it measures 

 the potential 

 difference be- 

 tween two 

 wires, should 

 be placed 

 across these 

 wires, and, 

 therefore, 

 should have 

 a high re- 

 sistance so as 

 to take but a 

 small current. 

 Since the 



Poimer 



Torsionol ond 

 conducting spring 



Movable element in 

 t±ie D'Arsonval type 



voltmeter's resistance is fixed, the cur- 

 rent through the meter will be propor- 

 tional to the E.M.F. in volts, so that, 

 like an ammeter, a voltmeter really oper- 

 ates in obedience to current variations. 



In the ammeter, the movable coil is 

 composed of a few turns of larger wire 

 than is used in the voltmeter. When de- 

 signed for small capacity, the total cur- 

 rent to be measured may be passed 

 directly through the coil. For heavy 

 currents, in excess of the ampere capacity 

 of the wire or in excess of full-scale meter 

 deflection, a portion of the current is 

 shunted through a low resistance circuit 

 called a shunt, which is paralleled in the 

 circuit of the movable element. Thus, by 

 using a suitable shunt, a current of any 

 magnitude may be measured. 



In the voltmeter, the moving element 

 consists of many turns of fine wire in 

 series with which there is a resistance. 

 This resistance is such that, when maxi- 

 mum voltage is applied, the current 

 through the movable coil is limited to the 

 amount necessary to give full scale de- 

 flection. 



Electrometers are instruments depend- 

 ing upon the mutual attraction between 

 opposite electrostatic charges. If a source 

 of E.M.F. is connected to two metallic 

 plates, they will take charges in propor- 



tion to their potential difference, and a 

 certain electrostatic attraction results. 

 If one of the plates is permitted to move, 

 the electrostatic capacity of the system 

 increases, thus increasing the amount 

 of the charges and the force of attraction. 

 This principle is employed in the con- 

 struction of electrostatic voltmeters, 

 adapted for the measurement of high 

 voltages. This meter is easily insulated, 

 of simple construction, requires no inter- 

 nal resistance wire, it consumes no cur- 

 rent on D.C. and practically none on A.C., 

 its deflections are independent of the 

 frequency, wave form, and stray magnetic 

 fields, and it indicates equally well on 

 direct and alternating current. 



The electrostatic voltmeter, shown in 

 Fig. 6, consists primarily of fixed and 

 movable metallic vanes of relatively large 

 surface, generally plane, but sometimes 

 curved. The two terminals are con- 

 nected, one to the fixed part and the other 

 to the movable part, which has a pointer 

 attached to give the deflections on a 

 graduated scale. The type of electro- 

 static voltmeter shown is designed for 

 potentials of 1,500 to 10,000 volts. 



In addition to the above standard types 

 of electrical meters, there is still quite a 

 number of other instruments designed for 

 special uses. These include: 



The frequency meter, or indicator, 

 used to determine the frequency or num- 

 ber of com- 

 plete cycles 

 per second of 

 an alternat- 

 ing current. 



The watt- 

 meter used for 

 measuring 

 directly in 

 watts the 

 power expen- 

 ded in a cir- 

 cuit. Watt- 

 meters are of 

 either the in- 

 dicating or re- 

 cording type. 

 Ohmmeters 

 used to give 

 directly the value, in ohms, of a resistance 

 that is being measured. 



The dynamometer, a moving coil 

 meter, used for measuring currents, 



Pointer 



Term. 



Movable vanes 

 Fig. 6 



Term. 



The voltmeter consists of 

 fixed and movable vanes 



