68 



ELECTRICAL MEASUREMENTS 



I 



production will be proportional to (10.3) 2 + (9.4) 2 + (10.3) 2 

 300.6, a change of 0.2 per cent. But the change in heat produc- 

 tion in wire a is from 100 to 106.1 or over 6 per cent, and that in 

 wire b is from 100 to 88.4, a change of over 11 per cent. To 

 take advantage of this total heat production the thermo-electric 

 arrangement may be altered. 



A fine Eureka wire may be soldered between a and b and a 

 copper wire between b and a'. The lead from 

 y\ * Ti to a is of copper, that from T z to a' of 

 , Eureka wire. 



Sectionalized Wires. High-frequency am- 

 2\ T Z meters in which the carrying capacity is in- 



FIG. 31. Thermo- creased by sectionalizing the wire, as shown 



function arrange- j p- 35 may contrary to general belief, 



ment for utilizing 



whole heat produc- show distribution changes due to the self and 



mutual inductions of the various parts. The 

 location of the leads A and B plays an im- 

 portant part in these changes; they should be symmetrically 

 placed. However, as all the sections of the wire contribute to 

 the deflection, that is, as the indication depends on the total 

 heat production, the resultant error is small though appreciable. 

 Wires of high resistance will eliminate the trouble. 



Use of Strips. A favorite method of obtaining large current 

 capacity is to employ a thin strip, of high resistivity, soldered 



FIG. 32. Roller's arrangement of terminals for high-frequency ammeter. 



between massive terminal blocks. With a high-resistance strip 

 the errors in this class of instruments are due to the effects of the 

 terminal blocks on the current distribution, the inductances of 

 the current paths being the determining factors. These effects 

 are avoided by the arrangement suggested by F. W. Roller, 

 shown in Fig. 32. Each part of the strip has about the same 



