( 400 ) 
Physics. — “The effect of the induction coil in telephonic apparatus” 
(22d part). By J. W. Gittay. (Communicated by Prof. P. 
ZEEMAN.) 
(Communicated in the meeting of December 28, 1901.) 
To arrive at our purpose we shall now try another method. Let 
us suppose an induction-coil without iron; in the primary wire is 
an undulatory current, in the secondary wire an alternating current is 
induced. If we now slide an iron core into that coil, the coefficient 
AGT 
of mutual induction will get B times greater, and if we tuke care 
that the strength of the current in the primary wire rests at the 
same value, notwithstanding the introduction of the iron, the induced 
Riots 
current will also be E times stronger than it was before. We shall 
se 
now investigate whether this B has the same value for all our coils. 
As, however, it would be difficult to make the intensity of the 
primary current after the introduction of the iron core equal to what it 
was before, we shall try to reach our aim in an indirect way. 
Fig. 9 shows how we set about for this experiment. To find the 
value of 2 for a coil with 3 primary layers, the primaries of 3 A 
and 3B were connected with microphone and battery in one circuit. 
a is a small coil with wire and an iron core in it, quite similar to 
that of the electro-dynamometer. Now the secondary of 3 A was 
connected with the coil a and the secondary of 3 B with the electro- 
dynamometer. So now we measured the current induced by 3B. 
By means of a commutator, left out of Fig. 9 for clearness’ sake, 
the electro-dynamometer was made to change places with coil a, so 
that 3 A was connected with the measuring instrument and 3 B 
with coil a. So now the current induced by 3 A was measured, 
and the strength of the current in the primary circuit had necessarily 
remained unaltered in both cases. 
Fig. 10 shows more elaborately how this experiment was arranged. 
If the 4 Morse keys are pressed down, 3 A is connected with the 
electro-dynamometer and 3B with coil a. If the keys are in rest, 
3 A is connected with coil a and 3 B with the measuring instrument. 
The result of these measurements are given in Table IV. For 
every measurement or comparison of 2 coils with each other 33 
turning points were read as formerly. As is seen from this table 
