Applied Science 831 



tance, and may be converted into visible or audible signals. This 

 is the principle of wireless telegraphy, and we see that the first 

 important requisite is to find some way of causing a number of 

 electrons to oscillate together, and thus initiate electric waves. 



Very rapid oscillations can be produced by the contrivance 

 called the Leyden jar (see Fig. 2 facing p. 825). The Leyden 

 jar is an electrical condenser by means of which electrical energy 

 can be stored. The Leyden jar consists essentially of a glass bottle 

 coated part of the way up, both inside and out, with tinfoil. If 

 now the inner coating be charged with electricity it induces a 

 charge of the opposite kind on the outer coating. The 

 glass separating the two pieces of tinfoil is thus put into a state 

 of electrical strain. The tinfoil acts as a means of evenly 

 distributing the charge over the surface of the glass. If the jar 

 be left like this it will preserve its two charges for a consider- 

 able time, but if the two coats be externally connected by 

 a wire, or the human body, or by any other conductor of elec- 

 tricity, a very rapid rush of electrons takes place, through the 

 conductor, from one coating to the other. But this rush 

 of electrons is so impetuous as to overshoot the mark. The energy 

 stored in the jar acts as a bent steel spring which is suddenly 

 released. It flies past the point of equilibrium and has to 

 return. It may have to perform several oscillations before it 

 finally comes to rest, when, in the case of the Leyden jar, the elec- 

 trons are equally distributed between the two coatings. The jar is 

 then said to be discharged. In order to discharge the jar it is not 

 necessary that the two metallic coatings should be actually con- 

 nected. A wire may be connected to each coating and the external 

 ends of the wires connected to two metal knobs. When now the 

 Leyden jar is charged, if the knobs are brought near to each other, 

 a spark will leap across the interval between them. The tension 

 set up by the different electricities of the jar is so great that it 

 will force the electrons across the air gap. But, from what we 

 have said, we see that this spark will not consist of a single leap 



