( 365 ) 
with sufficient accuracy that the deviations of the electro-dynamometer 
are proportional to the square of the mean current. 
To investigate which of the induction-coils named in Table I, 
connected in the usual way to a telephonic apparatus, would give the 
strongest induction-current I set to work in the following way: 
In the first place only coils with iron were compared with one 
another. In fig. 3, K and K 1 are the coils to be compared. I, II, HI 
and IV are Morse-keys which by means of a simple arrangement 
can be all pressed down at the same time. In the position of rest the 
keys themselves are connected with the upper contacts; cc is a double 
switch by which the current flowing to the electro-dynamometer 
can be opened or shut. R is a rheostat, shunted to the coil 
of the electro-dynamometer to reduce the deviations to a suitable 
size. As is easily seen from the figure, coil K1 is connected with 
the electro-dynamometer and with the microphone, as long as the 
keys are not pressed down; when the keys are pressed down, coil K is 
connected with the electro-dynamometer and with the microphone. 
All contacts not being broken or made by the four keys exactly 
at the same time when the coils were interchanged, care was taken 
that the commutator cc was always opened a little time before the 
keys were pressed down and before they were relinquished again, 
as otherwise strong induction-currents might find their way to the 
electro-dynamometer, which would give rise to remanent magnetism 
in the bundle of iron wires and a great modification of the zero- 
point. 
In the first place I have compared coil 3 A with 6A, 9A, 12 A 
and 15 A and after that again 6 A with 9 A, 9A with 12 A and 
12 A with 15 A. 
Table III (p. 366) shows how the experiment to compare 3 A 
with 6 A was made; in quite the same way the six other measu- 
rements were made. From the first four measurements I got for 
the proportion of the intensity of the induced currents giveu by our 
five coils: 
Coil: | 3A 6A 9A 12 A 15 A 
Intensity of the 
induced current: 
