710 
Proceedings of the Royal Society 
having coils so long as 180 feet of wire so thin as No. 36 — com- 
parative failure would result from the very great resistance offered 
to the current. We must try to do with coils offering an insignifi- 
cant amount of resistance. 
I find that the mode of arranging the parts of the telephone has 
a great influence on the efficiency, and the construction I have 
found up to the present time to he the best is as follows : — Taking 
a 3-inch horse-shoe magnet, I break off about half an inch from one 
leg ; the nick made to indicate the north end is usually at about 
this distance from the extremity, and the breaking of it there is an 
easy operation. I then round the unbroken end on a grindstone, 
and next gum a strip of thin paper round that leg, and upon the 
paper coil the desired length of silk-covered wire. Taking next a 
piece of wood 4x4 inches or 4 x 5 inches, and half an inch thick, a 
hole 2 inches in diameter is cut cleanly through it, and crossing the 
hole is fixed, in letter T fashion, another piece of wood 4x3 inches ; 
on this latter piece the magnet is laid with its south or coil-covered 
eud projecting centrally into the hole, and it is fixed down by one 
screw passing through a small cross piece of wood placed above it. 
This admits of after-adjustments being very readily made. The 
ferrotype plate, 2J inches in diameter, is laid as a cover over the 
outer aspect of the 2-inch hole, and secured there by the usual ring 
of wood which constitutes the ear or mouth piece. 
With such an instrument placed at A, fig. 2, and another tele- 
phone at A', I have experimented on the length of coil needed. 
Four or five feet of wire gave very satisfactory results, speaking 
being heard distinctly, and with sufficient loudness. I then tried 
shorter and shorter lengths, and found the volume of sound to 
become reduced certainly, but in a surprisingly small degree ; and 
at last I carried the shortening so far, that only 4J inches remained 
as a coil, and though the sound with this fragment was much 
reduced, giving the voice a far-away character, conversation could 
still be carried on. I did not think it necessary to push the reduc- 
tion further, having become convinced that no very great length of 
wire was needed for the coil to secure distinct hearing, at least for 
short lines, say of quarter of a mile or less. And for the present I 
have adopted a length of 9 feet of No. 28 wire (weighing 6 grains 
per foot length) as a satisfactory coil, presenting a resistance of only 
