( 404 ) 
The cause of this far from favourable result was due to the fact, 
as I afterwards found, that the microphone was not in good order 
when I made these measurements (they were the last I made with 
it). When it was connected with a telephone and a battery the 
former made a creaking sound, whilst no sound at all was made 
in the vicinity of the microphone. 
Now we know that the phenomenon, that the iron in our coils 
of higher order has smaller efficiency, than in those of lower order, 
is due to 2 facts: 
1. to the fact, that for the coils of higher order the secondary 
wire is wound on a wider cylinder, which causes many of the lines 
of force to cut twice the hollow space of that cylinder. 
2. to the weakening of the primary current when the iron is 
introduced, by the increase of the self-induction, which weakening 
is greater for the coils of higher order than for those of lower order. 
As is seen from the numbers of the 2"4 and 3rd rows on page 402, 
the cause mentioned sub 2 is the most important. This is also clear 
if we note that the primary coil is the only object with self-induction 
in the primary circuit, as the self-induction of the microphone, the 
microphone-battery and the very short connecting wires are practically 
equal to 0. 
So we have determined the influence of the iron on the intensity 
of the induced current in two ways: 1. by letting the harmful as 
well as the favourable influence of the iron act freely and 2. by 
bringing out only the favourable influence. 
I have determined the influence of the iron still in a third way, 
standing midway between the two above-mentioned methods. This 
method is represented in fig. 12 and fig. 13; the arrangement of 
the experiment is shown a little more in details in fig. 14. As 
fig. 12 shows, the two primary wires of coils A and B, which are 
to be compared, are connected in one circuit with the microphone 
and the microphone battery. The secondary of A is connected with 
the electro-dynamometer, the secondary of B remains open. The 
electro-dynamometer is now again the same as the one used for all 
the other experiments except for those of table V. In this way the 
current induced by A is measured. Now the secondary of coil B 
is connected with the electro-dynamometer, the secondary of A 
remaining open, as fig. 13 indicates. This change was made by 
means of the 2 Morse keys, as is shown in fig. 14: when the keys 
