.MEASUREMENT OF POTENTIAL DIFFERENCE 243 



C is insulated. The wire is in tension due to the spring S. 

 Normally the tension of the sections AB and CB is the same; 

 on the passage of the current, A B expands and the equality of 

 tension is restored by the rotation of B, which at the same time 

 moves the pointer. 



As the wires AB and CB are of the same material and mass, 

 the effect of variation of room temperature is compensated, even 

 though it be rapid. 



The whole movable system is mounted on a plate which can be 

 rotated about the point E; the zero adjustment is thus provided. 



In American practice, hot-wire instruments are not used on 

 switchboards. They are very useful in the laboratory, for being 

 adapted to the measurement of both alternating and direct cur- 

 rents, they can be used as transfer instruments in the calibration 

 of alternating-current ammeters and voltmeters. The alternat- 

 ing-current instrument may be compared, under normal condi- 

 tions of frequency and wave form, with the hot-wire instrument 

 and the latter calibrated immediately by use of the direct-current 

 potentiometer. 



When instruments of this type are first placed in circuit, enough 

 time should be allowed for them to come to their stable condition 

 before the readings are taken. 



The advantages of hot-wire instruments are: they have no 

 self-heating errors, are not influenced by local fields and are unin- 

 fluenced by changes of frequency and wave form;* the last is in 

 consequence of their low inductance. Their disadvantages are 

 that they are sluggish in action, the zero is unstable, they are 

 easily burned out by overloads, and the resistance of the voltmeter 

 is low. 



ELECTROSTATIC INSTRUMENTS 



In instruments of this class, advantage is taken of the electro- 

 ^tatic attraction existing between bodies charged to different 

 potentials. The magnitude of the force depends on the geometry 



* In strictness this remark applies only to instruments where the whole 

 current is taken through the hot wire. There are certain forms of instru- 

 ments which are equivalent to shunted ammeters in their construction. 

 They will show frequency errors at the very high periodicities used in radio- 

 telegraphy (see page 63), 



