( 361 ) 
the irregularity of the motion of the microphone had much decreased. 
The battery used on the microphone generally consisted of one 
Leclanché-cell. 
My original plan was to compare by hearing the effect of my 
different induction-coils. I therefore placed the vibrating tuning- 
fork before the microphone and connected by turns one of the two 
coils under comparison with the microphone and the telephone; of 
course in such a way that the primary wire was connected with 
the microphone and the battery, the secondary with the telephone. 
However, I soon found that nothing was to be expected of such 
an investigation, the comparison by hearing of the intensity of 2 
tones being exceedingly uncertain, unless the difference of intensity 
be very great. This is well known from the experiments of 
VOLKMANN and others"), which proved, that the ear does not hear 
any difference in the intensity of 2 tones, when it is smaller than 
about 30 per cent. 
So I was obliged to compare the induced currents, given by the 
different coils, to each other in another way, and for this purpose 
I made use of BELuati’s electro-dynamometer®). It is true, that 
even in this way we cannot yet obtain very accurate figures on 
account of the great irregularity with which such a microphone 
and such a tuning-fork work; yet in the following pages it will 
be seen, that by combining a sufficient number of readings the 
figures finally speak for themselves. 
The electro-dynamometer was provided with 2400 turns, thickness 
of wire 0.1 m.m., resistance 460 Ohm. The damping was brought about 
by means of a small circular piece of platinum in concentrated sulphuric 
acid. To determine the coefficient of damping the instrument was con- 
1) Wunpr, Physiologische Psychologie, 8rd edition Vol. I p. 364. This is also 
easily shown with a simple apparatus, indicated by Heymans, ‘Zeitschrift für Psycho- 
logie und Physiologie der Sinnesorgane”, Vol. XXI, p. 351, sub, 1. 
It has often astonished me that the innumerable improvements or rather modifi- 
cations applied to the construction of our magneto-telephones (especially in the first 
years after BELL’s invention) have scarcely any of them found their way. Although 
many of them were quite impossible, yet there were a few modifications from which 
something might have been expected. The cause of this is probably to be found in 
the above-mentioned fact: the reinforcement of the sound of the telephone arising 
from those improvements, will have been smaller than that minimum-limit, 
2) Wied, Ann. Neue Folge Vol. XXV, 1885, p. 325. As the instrument nowadays 
differs in shape so widely from the sketch, given with the above mentioned descrip- 
tion, I hereby give the drawing of the apparatus as it is made at present. The iron 
protecting-ring was not used by me. 
