196 Proceedings of Royal Society of Edinburgh. 
Telegraphic Communication by Induction by Means of 
Coils. By Charles A. Stevenson, B.Sc., F.R.S.E., 
M.Inst.C.E. (With Two Plates.) 
(Read March 19, 1894.) 
In 1892, I suggested that communication could he established 
between ship and ship by means of coils,* and as a trial of the 
system on a large scale has recently been made with the view 
of establishing communication between North Unst lighthouse, 
situated on Muckle Flugga, and the mainland (fig. 1), thence to 
the lighthouse station at Burrafiord, a distance of two miles, a 
record of the trials may he of interest to the Society. 
The induction of one spiral on another has been long known, 
but with a very strong battery current it has been found impossible 
to bridge a greater distance than 100 yards,! so that as a means of 
practical communication it was impossible. It has also been long 
known that communication could he established by means of 
parallel wires, and disturbances in wires no less than ten miles 
apart had been detected. For many years this system has been 
under discussion, and only last month a series of elaborate experi- 
ments at Loch Ness has been made by Mr Preece on this parallel 
wire system on the most approved methods ; but I trust to be able 
to show that the parallel wire system J; must give place to the 
method of communicating by coils. 
It is evident that if two coils are placed so that their axes are 
coincident, their planes being parallel, or if they be placed so 
that their planes are in the same plane, they will be in good posi- 
tions to expect electric currents sent in one to be apparent by 
induction in the other. For a given diameter, and where the 
electrical energy is small and the number of turns small, the first 
position is best, but where the energy is great and the number of 
turns great — in fact, when it is wished to carry the induction to 
many times the diameter of the coils — then it will be found that it 
is better to let the two coils be in the same plane, as, when the axes 
are coincident and the coilsa great distance apart in comparison 
with the diameter, the difference of distance from one side of the 
* Engineer, vol. Ixxiii. p. 292. t Jour, of Society of Arts, vol. xlii. p. 274. 
+ British Ass. Reports, 1886, p. 546 ; 1887, p. 611. 
