HOT-WIRE INSTRUMENTS 63 



It is hardly necessary to explain that one of the supports for the 

 wire must be insulated from the compensation plate, so as not to be 

 short-circuited by it.* 



In the case of a hot-wire voltmeter, the hot wire would be con- 

 nected in series with a high non-inductive resistance mounted inside 

 the case of the instrument, at the back of the compensation plate. 

 In an ammeter (unless for exceptionally small currents) the hot wire 

 would form a shunt across a resistance, which in instruments for 

 currents up to 200 amperes is contained in the instrument case, at 

 the back of the compensation plate, and in instruments for currents 

 above this value is separate from the instrument proper, special 

 connecting leads being provided for connecting the instrument across 

 the shunt. 



Hot-wire instruments are necessarily more or less sluggish in 

 their action, as the final temperature of the wire is reached gradually. 

 The makers provide an eddy-current damping plate (of aluminium), 

 as shown in Fig. 42. For an instrument not subject to mechanical 

 vibration, this damping arrangement is entirely useless. But where 

 violent vibration is present, the damping serves to check the rapid 

 vibrations of the pointer due to purely mechanical causes. 



The shaded circle shown in Fig. 42 to the left of the pulley corre- 

 sponds to the cross-section of the pillar supporting the front jewelled 

 bearing, which is shown removed. Another view of the damping 

 magnet is given at the right-hand lower corner of the figure. 



A serious disadvantage of the hot-wire type of instrument is its 

 liability to be fused by a momentary excess of current. 



Instruments constructed on the principles considered under (2) 

 and (3) in 31 are used to a considerable extent. As, however, 

 details of their construction are fully described in treatises on con- 

 tinuous currents, we shall not refer to them further here. As examples 

 of such instruments, we may mention Lord Kelvin's balances ; the 

 Weston standard voltmeter for alternating p.d.'s ; and the numerous 

 forms of "soft-iron" instruments, whose great advantage is their 

 cheapness. 



33. Electrostatic Instruments for Low Voltages 



On account of the importance of the electrostatic type of instru- 

 ment, we shall devote some space to a description of several examples 

 of this type. One of the best known is Lord Kelvin's multicellular 



* In the case, however, of ammeters intended for the measurement of large currents, 

 the hot wire is divided into two (or more) sections connected in parallel, by means of a 

 very thin strip (or strips) of silver foil attached to a point (or points) in the wire. The 

 ends of the wire are then at the same potential, and need not be insulated from each 

 other. 



