1893 - 94 .] C. A. Stevenson on Telegraphic Communication. 199 
averaging each 200 yards diameter, it was found that, with a 
phonopore, messages were sent with 5 dry cells. The resistance 
in primary being 30 ohms, and the resistance of secondary 260 
ohms, the current being *23 ampere, which, with 9 turns, gives 2 
ampere turns. (2) With a file as a make and break it worked 
with 10 cells, giving ’4 ampere or 3 ’6 ampere turns. 
Parallel Wire System. — (3) With a file as a make and break, 
and with parallel lines earthed, it was heard with 100 cells, giving 
IT ampere. Fig. 4 shows the variation with number of turns on 
one coil, the current and resistance remaining constant. 
The calculation of the diameter necessary to hear a given distance 
is simple, from the fact that the hearing distance is proportional to 
the J of the diameter of one of the coils, or directly as the 
diameter of the two coils, so that, with any given number of 
amperes and number of turns to hear double the distance requires 
double the diameter of coils, or double the number of turns, and 
so on. But this is within certain limits, for when the coils are close 
to one another the law does not hold. 
There is one point which seems to have been cleared up by these 
trials, which has even this month been a subject of discussion in 
London, and that is, whether or not the parallel wire system is 
actuated by induction or conduction, and there is little room for 
doubt, from the fact that both circuits were insulated, and was thus 
a case of pure induction, that to a large extent it is induction ; in 
fact, that they act together, and it will depend how the ends are 
earthed, or, in short, what is the distance bridged in comparison to , 
the breadth of base, which predominates. Where the wires are 
long in comparison with the distance bridged, conduction will be 
the main working factor, but when the base is small, and the 
distance bridged is large in comparison, induction will be the main 
factor, and the number of turns then increases the effect.. 
The primary coil was insulated in the Murrayfield trials, as at 
Muckle Flugga it must be so, the impracticability of making and 
maintaining the sea connections necessitating this, and the secondary 
was earthed, as is most convenient at Muckle Flugga. When the 
secondary was also made a complete insulated metallic circuit, there 
seemed to be little difference in the result. 
There is one other point to which reference must be made. Mr 
Preece has been repeating the experiments brought before this 
