WIBELESS TELEGrEAPHY. 
119 
pass across the gap. I connect to the two sides of the receiver, wires 
from the dynamo in the base of the building, between which a con¬ 
siderable electrical pressure exists. Nothing will happen until a spark 
passes, but immediately this occurs the obstruction formed by the air 
gap “ breaks down,” and the well-known brilliant electric arc is 
formed between the carbon points. 
(Experiment shewn :—Wires from dynamo connected one to each 
side of the receiver gap. On pressing the “transmitter” key a few 
feet away the electric arc immediately started). 
The assistance of a dynamo is not absolutely necessary to shew 
you all, that electrical oscillations are produced in this receiving ap¬ 
paratus owiug to the primary oscillations of the transmitting appar¬ 
atus. For if I connect a small “vacuum tube” across the gap of 
the receiver, you will see the tube glow strongly when sparks are 
caused to take place across the transmitting apparatus. 
(Experiment with vacuum tube shewn). 
It is not, perhaps, an easy thing at first to grasp, that when electrical 
oscillations are produced, electro-magnetic (Hertzean) waves are 
thrown out at right angles to the line of oscillation. These waves 
in their turn are able to excite similar (but less intense) electrical os¬ 
cillations in a suitably constructed receiving apparatus at a distance. 
These “wave models” may assist you. I am indebted to Pro¬ 
fessor Sylvanus Thompson for the loan of the large model. It is 
the model he exhibited at the ftoyal Institution not long since. The 
communication of an oscillation from one swinging body to another 
at a distance becomes beautifully evident, the intermediate mechan¬ 
ism being in the model a long row of leaden balls suspended in a 
peculiar manner. 
Captain Kennedy lent me the smaller model, whose action is sim¬ 
ilar, though there are in his model one or two ingenious improve¬ 
ments. These are, I believe, the only two models of the kind in 
existence. 
(Models shown and action explained in more detail). 
The mechanism that transmits electrical oscillations is invisible to 
us. “ Ether ” is the name given to the medium in which the wave 
motions take place, and there is no doubt that by some transverse 
undulations in this medium, the electro-magnetic (Hertzian) waves 
as well as light waves are conveyed. No one knows what the actual 
“mechanism” of the ether is (if one may use such a term) but know¬ 
ledge is gradually being gained on the subject. 
The experiment just shewn you serves very well to indicate the 
nature of the apparatus required for transmitting signals by means 
of these Hertzian waves. Besides the transmitting apparatus pro¬ 
ducing the waves we require a detector of such waves. Moreover 
the detector will produce a more visible signal of it and serves to bring 
into action some local source of energy, so that the fact of the arrival 
of the wave may be demonstrated in a more energetic manner. For 
instance, the local source of energy in this experiment was a dynamo in 
