1889.] 071 the Discharge of a Leyden Jar. 415 



is moved. Take the simplest meclianical analogy, that of the 

 vibration of a loaded spring, like the reeds in a musical box. The 

 stiifer the spring, and the less the load, the faster it vibrates. Give 

 a mathematician these data, and he will calculate for you the time the 

 spring takes to execute one complete vibration, the " period " of its 

 swing. [Loaded lath in vice.] 



The electrical problem and the electrical solution are precisely 

 the same. That which corresponds to the flexibility of the spring, is 

 in electrical language called static capacity, or, by Mr. Heaviside, 

 permittance. That which corresponds to the inertia of ordinary 

 matter is called electro-magnetic inertia, or self-induction, or, by Mr. 

 Heaviside, inductance. 



Increase either of these, and the rate of oscillation is diminished. 

 Increasing the static capacity corresponds to lengthening the spring ; 

 increasing the self-induction corresponds to loading it. 



Now the static capacity is increased simply by using a larger jar, 

 or by combining a number of jars into a battery in the very old 

 established way. Increase in the self-induction is attained by giving 

 the discharge more space to magnetise, or by making it magnetise a 

 given space more strongly. For electro-magnetic inertia is wholly 

 due to the magnetisation of the space surrounding a current, and this 

 space may be increased or its magnetisation intensified as much as 

 we 2)lease. 



To increase the space we have only to make the discharge take a 

 long circuit instead of a short one. Thus we may send it by a wire 

 all round the room, or by a telegraph wire all round a town, and all 

 the space inside it and some of that outside will be more or less 

 magnetised. More or less, E say, as it is a case of less rather than 

 more. Practically very little effect is felt except close to the 

 conductor, and accordingly the self-induction increases very nearly 

 proportionally to the length of the wire, and not in proportion to the 

 area inclosed : provided also the going and return wires are kept a 

 reasonable distance apart, so as not to encroach upon each other's 

 appreciably magnetised regions. 



But it is just as effective, and more compact, to intensify the 

 magnetisation of a given sjDace by sending the current hundreds of 

 times round it instead of only once ; and this is done by inserting a 

 coil of wire into the discharge circuit. 



Yet a third w^ay there is of increasing the magnetisation of a 

 given space, and that is to fill it with some very magnetisable 

 substance such as iron. This, indeed, is a most powerful method 

 under many circumstances, it being possible to increase the magneti- 

 sation and therefore the self-induction or inertia of the current some 

 5000 times by the use of iron. 



But in the case of the discharge of a Leyden jar, iron is of no 

 advantage. The current oscillates so quickly that any iron introduced 

 into its circuit, however subdivided into thin wires it may be, is 

 protected from magnetism by inverse currents induced in its outer 



