( 358 ) 
When shortly after attempts were made to use the microphone 
in practical telephony, it was soon made evident that on lines of 
some resistance the sound emitted was a great deal too faint. The 
slight changes of resistance taking place in the microphone were 
of so little importance when compared to the great resistance of the 
line, that only a slight, practically useless variation of the current 
appeared in the line. 
It was then that Eprson, and at about the same time HooRwEG, 
found a simple means to overcome this obstacle: the microphone 
was no longer placed in the line but, with the battery, in the 
primary circuit of a small induction-coil. The ends of the secondary 
wire of that coil were connected with the two lines or with line 
and earth. By an exact selection of the number of secondary turns 
the undulating current of the battery was transformed into an 
alternating current of much higher potential, and telephonic messages 
could be sent over lines of much higher resistance than formerly. 
Except for the use of the telephone at distances of but a few 
hundred metres, not a single telephonic apparatus nowadays is found 
without induction-coil. 
The induction-coils generally used for telephonic purposes, are as 
a rule provided wich four layers of primary wire, each layer con- 
sisting of about 90 turns; the thickness of the wire generally is 
0.5 mM. The secondary is usually made of 0.12 mM. thickness and 
consists of about 3000 turns. In the coil is an iron core. 
Practice has shown that such a coil in most cases gives the best 
results. 
The literature about the choice of the induction-coil is very scanty, 
very few experiments seem to have been made on this subject. 
In the well-known book of Preece and SrugBs “a Manual of 
Telephony” is a table which gives the results of some comparative 
experiments made by Mr. ABREZOL for the Swiss Telegraphic- 
Department. The microphone used by him was that of Blake, 
then in general use; it was connected respectively with ten different 
induction-coils and then the observed intensity and clearness” 
was noted down. The word “clearness” probably denotes the degree 
of articulation. 
The intensity and “clearness” of one of the coils were both 
indicated by 1; for another coil, e.g. 0.3 was found for the intensity 
and 0.9 for the “clearness”. 
The small amount of what is communicated about these experiments 
in the book of Preece & Srusss makes me think, that the figures 
ie 
