HOT-WIRE INSTRUMENTS 61 



and traversed by the current to be measured. The stress between 

 the two coils is proportional to the product of their currents, i.e., the 

 currents being the same, to the square of the current. (3) A coil 

 acting on a feebly magnetized core. So long as the magnetization is 

 kept sufficiently low, the permeability may be regarded as constant, 

 ami hysteresis negligible. Thus for given relative positions of coil 

 and core the stress between them will be proportional to the mean 

 square of the current. 



Passing now to the measurement of p.d.'s, all the above arrange- 

 ments may be employed, provided the instrument circuit is arranged 

 so that the current in it is proportional to the p.d., and independent 

 of the frequency of the p.d. This latter condition involves negligible 

 reactance in comparison with the resistance of the instrument circuit. 

 There is, however, a further arrangement available for the measure- 

 ment of p.d.'s, which is by far the most satisfactory. It consists of 

 a fixed and a movable system of plates, the two systems being in 

 connection with the points across which it is desired to measure the 

 p.d. For a given relative position of the two systems, the electric 

 field intensity will at any instant be in simple proportion to the 

 p.d. But since the stress between the plates varies in proportion to 

 the square of the electric field intensity, it follows that the mean 

 value of this stress will be proportional to the mean square of the p.d. 



We shall now describe a few important types of alternating 

 current instruments whose action is based on the above principles. 



32. Hot-wire Instruments 



The prototype of all hot-wire instruments is the at one time well 

 known and widely used, but now entirely obsolete, Garde w volt- 

 meter. In Fig. 42 is shown a modern type of hot-wire instrument, 

 designed by Messrs. Hartmann and Braun, and manufactured in this 

 country by Messrs. Johnson and Phillips. The " hot wire " is a thin 

 wire of platinum-silver stretched between two supports, one of which 

 is a fixed pillar, while the other is provided with an adjustment for 

 varying the tension of the wire, and so altering the zero position of 

 the pointer. Platinum-silver is used as the material for this wire 

 because of its high resistivity and non-oxidizable property, which 

 enables it to stand the action of comparatively high temperatures 

 when freely exposed to the air. The " hot wire " is maintained in a 

 state of tension and made to sag by the side pull exerted on it by 

 a phosphor-bronze wire attached to a point about its middle. The 

 phosphor-bronze wire (which is not traversed by any current) is in 

 its turn maintained in a state of tension by a silk fibre attached to 

 it and passing round the circumference of a small pulley, on whose 



