1893 - 94 .] C. A. Stevenson on Telegraphic Communication. 197 
coil, say top of primary coil to top and bottom of secondary, 
becomes almost a vanishing quantity ; whereas, when the coils are 
lying on their side in the same plane, the difference of distance from 
back of primary to back of secondary, and from front of primary 
to front of secondary, does not fall off so fast, and consequently is 
more efficacious. Besides, it becomes impracticable to erect coils 
of large diameter with their planes vertical, but it is easy to lay 
them on their sides. It is also impracticable to introduce a core in 
these large coils, although the effect would thereby be intensified, 
and where compactness is necessary a core is advantageous. 
A number of experiments were made in the laboratory to dis- 
cover the laws of the action of coils on each other, with a view of 
calculating the number of wires, the diameter of coils, the number 
of amperes, and the resistance of the coils that would be necessary 
to communicate with Muckle Flugga, and, after a careful investiga- 
tion, it was evident that the gap of 800 yards could, with certainty, 
be bridged by a current of one ampere with coils of nine turns of 
N’o. 8 iron wire in each coil, the coils being 200 yards in diameter. 
Two coils, about 850 yards centre to centre, were erected at 
Murrayfield (and I may here thank Mr Gibson, telegraph engineer 
of the General Post Office, and his staff, and also Mr Asher of the 
National Telephone Company, and Messrs Clement & Francis of 
the North British Eailway, for their valuable assistance), one coil 
being on the farm of Damhead, and the other on the farm of 
Saughton, and as nearly as was possible on a similar scale, and the 
coils of similar shape,* as was wished at Muckle Flugga. 
On erecting the coils, communication was found impossible, owing 
to the induction currents from the lines from Edinburgh to Glasgow, 
the messages in these lines being quite easily read, although the 
coils were entirely insulated and were not earthed. The phonopore 
which the North British Railway Company have on their lines 
kept up a nearly constant musical sound, which entirely prevented 
observations. On getting the phonopore stopped, it was found that 
100 dry cells, with 1*2 ohms resistance each and 1’4 volts, gave good 
results, the observations being read with great ease in the secondary 
by means of two telephones. The cells were reduced in number 
* Shape of coil, circle, square, or rectangle, &c., is not very material, and in 
practice must be altered to suit circumstances, as for instance in a ship 
(fig. 2). 
