234 ELECTRICAL MEASUREMENTS 



The sample shown at X is placed alongside a scale divided into 

 100 equal parts; the graduations therefore represent percentages 

 of the total length of the scale. 



Consider X to be of uniform cross-section and 100 per cent, 

 conductivity, that a and 6 are set at and 100, respectively, and 

 that the resistance of P equals that of X; for balance c and d 

 must be set so that M = N. Now suppose that the sample at X 

 is changed for one of the same diameter, but of 50 per cent, con- 

 ductivity; the length required to balance P, contacts c and d 

 remaining fixed, will be only 50 per cent, as great as in the first 

 case, and 6 must be moved along the percentage scale to the 

 50 per cent. mark. 



However, the samples X vary in diameter, while the resistance 



of P is fixed; consequently the ratio j^- must be variable, so 



that it may be made to correct for the cross-section of the sample. 

 To obtain the relative diameters of wires it is more accurate and 

 convenient to weigh samples of the same length than to caliper 

 them; accordingly, all samples are cut to a length of 38 in. in a 

 special machine and weighed. The contact c is then set at the 

 graduation corresponding to the excess or defect in the weight, 



referred to a sample of correct size, thus making -- P equal to 



the resistance of a sample of 100 per cent, conductivity length 

 0-100 on the percentage scale and of the same diameter as 

 X. The bridge is then balanced by moving 6, and the con- 

 ductivity is read from the scale. 



Several standards are provided; they are removable, and by 

 the use of the taps e, /, g, each has a range of three consecutive 

 numbers on the B. & S. gage. For rapid work the stock of 

 samples must be kept at the temperature of the testing 

 apparatus. 



The coarse adjustment of the slider b is effected by the handle 

 projecting toward the front of the bridge; the final balancing is 

 made by turning the milled head at the front. 



The percentage scale is seen at the back of the instrument, the 

 excess of weight scale at the right hand. 



The instrument is covered by a metal-lined wooden case, which 

 serves to keep the temperature constant; the reading is made 



