THE AID OF THE ACHROMATIC FRINGES 



49 



tested, should, from the measurement of the current induced in the secondary 

 in question and in the absence of other mutual inductions, give the constants of 

 the secondary. As many of the coils to be tested were of small internal diam- 

 eter, the primary was wound on a long, thin iron tube, fine wire being neces- 

 sary. The dimensions were for the iron tube : diameter outside, 0.63 5 cm. ; inside, 

 0.470 cm.; length, 55 cm.; walls, 0.08 cm. thick. For the helix : diameter out- 

 side, 0.7 cm. ; wire, 0.034 cm. in diameter; n\/l\ = 21 turns per linear centimeter. 



If L is the coefficient of induction per turn of primary, the total induction is 

 (i) B = Lnii 



Hence the electromotive force induced in the secondary becomes 



If the field of the secondary per unit current is put 



for M 2 //2 turns per linear centimeter, and r 2 the resistance of the coil and its 

 circuit, we may compare any two coils by the equation 



e z and e' z being the electromotive forces, and i z , i z the currents induced in the 

 two secondaries in question. Thus 



A measurement of s and the total resistance of the secondary circuit should 

 therefore give us the unit field within, when the winding is unknown. It is 

 assumed that the resistance r z added to the secondary circuit is so large that 

 the inductive resistance vanishes, so that n = r'z. 



Thrusting this coil tester or primary in the helix No. D heretofore used 

 (H = 1132') the results of figure 52 were obtained. R is the resistance added 

 to that of the coil and the inductive resistance, both of which are small 

 relative to R. 



Unfortunately, the constant resistance of the circuit (about r = S4o ohms) 

 was not carefully taken; but the products (R+r) i, show that with an addi- 

 tional resistance above 1,000 ohms the inductions may be disregarded. 



The coil-tester was now thrust through a variety of helices, as follows: 

 (/ length of helix, d internal diameter, n/l turns per cm. of length per layer, 

 S number of layers, d diameter of wire) : 



The last column shows the ratio of the field within and the current, i, in 

 amperes, if H =^n/l is considered sufficient. 



Figure 53 gives the results from observations read off for these coils at the 

 4 



