PHYSICS: C. BARUS 
217 
If L is the coefficient of induction per turn of primary, the total induction is 
B = LNii (1) 
Hence the electromotive force induced in the secondary becomes 
e = LNiN2{dii/dt) (2) 
If the field of the secondary per unit current is put 
H, = ^-wNt/h (3) 
for Nilh turns per linear centimeter, and r the resistance of this coil and its 
circuit, we may compare any two coils by the equation 
^1 _ ^ih _ ^JlI (a\ 
62 and e'2 being the electromotive forces, and and i'2 the currents induced in 
the two secondaries in question. Thus 
H2 _ iir 2 /h _ S2/I2 /px 
ti2 tir2lh S2/I2 
it being assumed that the resistance r in the secondary circuit is made so large 
that the inductive resistance vanishes. 
The coil tester was now thrust through a variety of helices, differing in shape 
and construction. 
Figure 7 gives the results read off for these coils at the vibrator (5), when a 
high resistance was added to the circuit and Ri constant. The relation of 
Ha and 5 as seen in figure 7, is hnear for coils W, B, G, which were about of 
the same length and wound on non-conductors or split brass. The result for 
coil D, which was about 1.8 times longer is low, probably owing to mutual.in- 
duction, as this coil was wound on a thick brass tube without fissure. Sim- 
ilarly it made very little difference whether the two helices wound side by side 
throughout G, were used in parallel or in series. Finally a number of single 
layer coils of lengths 10, 20, 30, 40 cm. were wound on stout glass tubing and 
compared with B. Provided care was taken to prevent the induced currents 
from heating the telephone, the band width 5 increased throughout propor- 
tionally to the number of turns of wire in the secondary and under good con- 
ditions at a rate of about 5 = 0.06 scale parts per turn of wire. 
