THE MEASUREMENT OF CURRENT 63 



The Parallel-wire Ammeter. In this form of instrument the 

 conductor is a group of several straight wires of the same length 

 and diameter and so fine that changes of resistance due to skin 

 effect are negligible; they are parallel in direction and usually 

 are equally spaced. 



A typical arrangement of this sort is shown in Fig. 29. The 

 current is led in through the heavy terminals which are perpen- 

 dicular to the active wires and therefore exercise no inductive 

 effect on them. The distributing terminals at the ends of the 

 parallel wires have a negligible impedance. 



In a perfect instrument all the wires will have the same 

 resistance and a direct or low-frequency current will divide 

 equally between them, all 

 inductance effects being neg- 

 ligible. At very high fre- 

 quencies the distribution of 

 current between the wires is 

 determined by the induc- 

 tances, self and mutual, 

 rather than by the resis- 

 tances, SO that it may be FIG. 29. Parallel-wire high-frequency 



j-ff L f j.1. j- ammeter, 



very different from the di- 

 rect-current distribution; that is, the current distribution may 

 be a function of the frequency. 



It is essential that the wires be of uniform resistance. Lack of 

 uniformity may be due to variation in hardness or small varia- 

 tions in the cross-section. These things will not affect the induc- 

 tances, so the wires may carry equal currents at high frequencies 

 but very different currents at low frequencies where the resistance 

 effects preponderate. 



If the indicator is attached to one wire, as in Fig. 29, the 

 magnitude of the error will depend on which wire is used. It 

 will be decreased if the arrangement is such that all the wires 

 contribute to the functioning of the indicator, for if the current 

 in one wire is decreased, that in the others must correspondingly 

 increase and the change in the total heat production is much less 

 than that in any one wire. Bellinger cites the case of a seven- 

 wire instrument, the indicator of which was operated by one of 

 the wires somewhat distant from the others. At 100,000 cycles 



