through Exhausted Tubes without Electrodes. 329 



depends upon the size of the leads ; if all the circuit were 

 available for this coil one turn would give the largest electro- 

 motive force, because though for a given rate of change of the 

 current in the primary the effect on the secondary increases with 

 the number of turns, the rate of change of the current varies 

 inversely as the self-induction of the primary, so that if all the 

 circuit is in the coil C, since an increase in the number of turns 

 will increase the self-induction of the circuit faster than the 

 mutual induction, it will diminish the electromotive force 

 round the secondary. In practice, however, it is not possible 

 to have the whole of the wire connecting the coatings of the 

 jar in the coil C ; and in this case an increase in the number 

 of turns may increase the mutual induction more than the self- 

 induction, and so be advantageous. The best result will be 

 obtained when the self-induction in the coil C is equal to that 

 of the remainder of the circuit. It is very easy to find by 

 actual trial whether the addition of an extra turn of wire is 

 beneficial or the reverse. The brightness of the discharge 

 depends upon the time of the electrical oscillations as well as 

 upon the magnitude of the electromotive force. Thus, in an 

 experiment to be described later, the brilliancy of the dis- 

 charge was increased by putting self-induction in the leads 

 which, though it diminished the intensity of the electromotive 

 force, increased the time-constant of the system. When the 

 discharge-tube was square and the coil C had also to be square, 

 it was found most convenient to make it of glass tubing bent 

 into the required form and filled with mercury. When, how- 

 ever, the discharge was required in a bulb, the primary coil was 

 made of thick gutta-percha-covered copper wire wound round 

 a beaker just large enough to receive the exhausted bulb. 

 There is sometimes considerable difficulty in getting the first 

 discharge to pass through the bulb, though when it has once 

 been started other discharges follow with much less difficulty. 

 The same effect occurs with ordinary sparks. It seems to be 

 due to the splitting up of the molecules by the first discharge; 

 some of the atoms are left uncombined and so ready to con- 

 duct the discharge, or else when they recombine they form 

 compounds of smaller electric strength than the original gas. 

 When the discharge was loath to start, I found the most 

 effectual way of inducing it to do so was to pull the terminals 

 of the Wimshurst far apart and then, after the jars had got 

 fully charged, to push the terminals suddenly together. In 

 this way a long spark is obtained, which, if the pressure of 

 the gas is such that any discharge is possible, with the means 

 at our disposal will generally start the discharge. 



