

Use of Mutual Inductometers. 503 



The coil (A) is then tested in the bridge, and if the values 

 obtained for X! and T' are not those given by equations (18) 

 and (19), a small amount of inductance is added to R or S 

 until X' and T / are read correctly. When this adjustment 

 has been made once for all, it will be found that any other 

 effective resistance will be measured correctly. 



It may be mentioned that a well proportioned 0*1 henry 

 coil of 7-strand wire, each strand being of 0*2 mm. diameter 

 and separately insulated, should show practically no variation 

 in effective inductance and resistance due to skin effect at 

 1000 ~ per sec.*; its distributed capacity should be less than 

 0*0001 mfd. If a coil when tested at various frequencies 

 shows percentage variations in its effective resistance which 

 are double those in its inductance, then these variations can 

 be accounted for by distributed capacity ; but if the variation 

 of the effective resistance is at a greater relative rate, then 

 we must look for additional causes such as leakage or skin 

 effect (or errors in the bridge). 



When the self inductances of the inductometer coils are 

 high, their distributed capacities may cause errors at the 

 higher frequencies. The effects of the capacities of the coils 

 Lj and L 2 (fig. 1) are got rid of by the method of preliminary 

 balancing ; unfortunately this does not eliminate the effect 

 of the capacity k of the external inducing coil. If its 

 resistance and self inductance are p and x respectively, then 

 for an equal arm bridge, instead of equations (2) and (4) 

 we have 



Q = P + 2co 2 k P M-co 2 kW .... (20) 



and N = 2M(l+o> 2 ^), (21) 



where k is in farads. 



The error in Q due to the second term in (20) is usually 

 the most serious. Both errors can be approximately elimin- 

 ated by connecting a small capacity of suitable amount across 

 the points c, d (fig. 1). 



§ 4. JWidl Method in Iron Testing. 



The use of a mutual inductometer affords a null method 

 for the magnetic testing of iron ; this is analogous to the 

 self inductance method which Max Wien investigated very 

 thoroughly some years ago f, and there are cases in which it 



* See M. Wien, Ann. der Physik. p. 1 (6), 1904, and Dolezalek, /. c. 

 p. 1142 (12) 1903. ' 



t Ann. der Physik, p. 859 (66), 1898. 



