Inductance hy the Aid of a Vibration Galvanometer. 501 



Values may be given to L. Thus, i£ L be changed to some 

 sub-multiple L/q, the scale readings for n will merely have to 

 be multiplied by q throughout, provided that P is kept 

 constant. Perhaps a better way is to keep L unchanged, 

 and alter P + Q and II 4- S each in the ratio 1 : q, the 

 alterations being made in P and R only, leaving Q and S as 

 before ; from formula (6) it will be seen that, for the same 

 scale reading, n will become qn. The value of the variable M 

 does not require to be known. The following example, giving 

 values which I have actually used, may be of practical interest. 

 P = 25 ohms, Q = 5 ohms, rl+S = 4 ohms, L = 0'1066 henry. 

 The graduation of the slide-wire corresponds to Table I. 



Table I. 



1 





Temperature - 



11. 



s. 



coefficient 







of 11. 



— . per see. 



ohm. 



°/o P e1 ' degree C. 



10 



0-642 



-0-43 



15 



0-608 



-0-32 



20 



0-562 



-0-21 



30 



0-471 



-0-08 



40 



0-392 



-o-oi 



50 



0-320 



+0-04 



60 



0-260 



+0-08 



70 



0-213 



+0-09 



80 



0-175 



+0-10 



90 



0-147 



+ 0-11 



100 



0-126 



+0-11 



110 



0-107 



+0-12 



120 



0-090 



+0-12 



Since M~ 0*00313 S, its range of variation will be from 

 about 2 millihenries at n = Q down to about 0*28 millihenry 

 at 71 = 120. 



This case is usually sufficient to discriminate the actual 

 frequency to which the galvanometer is responding, the 

 exact frequency of the source being determined by com- 

 parison with a standard fork or stroboscopically. 



Case 2. — If P be not exactly known, a temperature cor- 

 rection may be applied to the scale readings. Thus in the 

 above example, in which P was entirely of copper, the 

 temperature coefficients of n at various points of the scale are 

 shown in the third column of Table I. It will be noticed that 

 the correction may become large only below 30^- per second. 



II t'he variable M has been accurately calibrated, p may be 

 obtained by the equation 



■ (q+sxr+S) /sy 



