194 Mr, W. H, Preece [May 23, 



WEEKLY EVENING MEETING, 



Friday, May 23, 1879. 



William Spottiswoode, Esq. D.C.L. L.L.D. President E.S. 

 Vice-President, in the Chair. 



W. H. Preece, Esq. M. Inst. C.E. M.B.L 



Multiple Telegraphy. 



Many of you, in your rural rambles, may, during a moment of reflec- 

 tion or tlioughtlessness, while standing on the parapet of a rustic 

 bridge, have dropped a stone into the river flowing gently below, and 

 have noticed the rings of waves projecting outwards in ever increasing 

 circles. If at the same moment a hungry fish, in his desire to satisfy 

 a natural craving, should dart or snatch at a fly on the surface of the 

 water, a second series of rings of waves would be produced, and in 

 such a case a careful observer would notice that where crest of wave 

 meets crest of wave there is a higher crest formed, and where hollow 

 meets hollow a deeper hollow is formed. This super-position of wave 

 on wave is called interference, and the interference of undulations 

 plays a most important part in the phenomena of sound, of light, and 

 of electricity. 



In electricity, wave upon wave can be super-imposed, either in 

 waves flowing in the same direction or in waves flowing in opposite 

 directions. The usual indication of the presence of electric waves is 

 either by the attraction of a magnet or by the deflection of a needle. 



Here is a needle that is subjected to deflection by the passage of 

 such waves of electricity. I send a current of positive electricity 

 through the wire surrounding the magnet to M^hich this needle is 

 attached, and you see I get a certain deflection ; I increase the 

 strength of that wave by super-imposing another wave, and you 

 see I increase the amount of the deflection. In the same way, 

 if I reverse the direction of the current and send a negative current, 

 I get a wave of a certain strength, and if I double the strength of the 

 current I get a stronger deflection. But if, instead of super-imposing 

 one wave on the other in the same direction, I send one wave in one 

 direction and the other wave in the opposite direction, we get 

 neutrality. This neutrality is a phenomenon corresponding very 

 much to silence in sound or to darkness in light. 



Advantage is taken of this neutrality in duj^lcx and quadruplcx 



