Page:Popular Science Monthly Volume 92.djvu/967

 Popular Science MontJily

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��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

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