180 



ELECTRICAL MEASUREMENTS 



successively interchanging the coils with a and finding the cor- 

 responding balance points, a series of length having equal resist- 

 ances may be set off on the slide wire. 



Thomson Bridge Method for Calibrating a Slide Wire.- 

 Connections are made according to Fig. 102. CD is a resistance 

 which determines that of the steps into which the slide wire is to 

 be divided. It is connected in series with the slide wire by a 

 resistance R which is considerably greater than that of the wire 

 and which can be short-circuited. The coils M, N, m, n, should 



M 

 fulfill the condition - 



If this condition is exactly ful- 



M N 



filled, the settings will be independent of the resistance between 

 D and Zij to make sure that this is so h is set at any point on the 



slide wire, R is short-circuited 

 and a balance obtained by 

 adjusting Z 2 The short-cir- 

 cuit around R is then re- 

 moved. The balance should 

 remain undisturbed and this 

 should be true whatever the 

 setting of li. A more severe 

 test is to have a break at R, 

 that is, to make R = c . If 



FIG. 102. Diagram for Thomson- 

 bridge method for calibrating a slide 

 wire. 



this test is satisfactory, the 



!i may be set at various points along the wire and bal- 

 obtained by moving Z 2 . The resistance of the sections 



slider 

 ances 



will then be r* 2 - n = r c o ( TF ) * 



Calibration Corrections. If it appears from the data obtained 

 by any of these methods that the wire is not uniform, it is best 

 to replace it by another. If circumstances preclude this, the 

 calibration observations may be reduced as follows: 



It is desired to find the corrected values of the readings to be 

 inserted in the formulae already given for the slide- wire bridge; 

 that is, the readings on a uniform wire of the same total resistance 

 as the wire actually used. 



The lengths of the sections of equal resistance are plotted 

 as ordinates. The positions of the lower ends of the sections 

 are taken as abscissae. A smooth curve is then drawn through 



