1892 - 93 .] Mr Stevenson: Induction through Air and Water. 27 
seems to be no practical difference to the efficiency of the instrument 
caused by air or by water, salt or fresh. 
The action of the instrument appears to be that the break, which 
is well known as a much more marked phenomenon' than the 
“ make,” if broken sufficiently rapidly, induces a current in the coil, 
which the core intensifies immensely, and the effect on the instrument 
is apparently simply affected by the number of amperes. 
I found that the distance at which the instrument was audible in 
air was proportional to the current in amperes, so that where the 
external resistance was great in comparison with the resistance of the 
battery, doubling the wire doubled the current, and so the distance. 
This is evidently the case in water, as is seen by the observations 
(fig. 3). The volts were measured by one of Cardrew’s voltmeters. 
It will be readily seen that the distance due to a certain number of 
amperes is only applicable to the particular instrument in use, and is 
not the limit, as a more sensitive instrument could be made. The 
one used was 3 feet in length, with about 2000 turns of wire, varying 
from to 3 ^^ inch in diameter. 
The sound in a Bell telephone with this instrument was almost 
deafening with 3J fathoms. The result did not appear so effective 
with a closed circuit of a small magneto machine as with the current 
produced by dry cells being made and broken. 
This electro-magnet system of induction, in contradistinction to the 
parallel wire or electrostatic systems, has no earth connection, being 
entirely insulated, and must therefore be a case of true induction 
through air or water. 
