Exciting an Induction-coil. 391 



De Meritens machine so driven is about 1300 revolutions per 

 minute. 



In this arrangement the "make" currents are of course 

 alternately in one direction and in the other, as also are the 

 "break" currents; so that the discharge appears to the eye 

 during the working of the machine the same at both termi- 

 nals of the tube. 



The advantages of the method are : — first, the fact that, as 

 the machine effects its owxi make and break, both the contact- 

 breaker and the condenser of the induction-coil can be dis- 

 pensed with ; secondly, that the breaking of the primary, 

 and consequently the delivery of the secondary, currents is 

 perfectly regular ; thirdly, that the quantity of the currents 

 in the secondary is very great. With a 20-inch coil by Apps 

 I have obtained a spark about 7 inches in length, of the full 

 thickness of an ordinary cedar pencil. But for a spark of 

 thickness comparable at least with this and of 2 inches 

 length an ordinary 4-inch coil is sufficient. 



Owing to the double currents, this spark consists of a bright 

 point at each terminal, and a tongue of the yellow flame, such 

 as is usually seen with thick sparks from a large coil, issuing 

 from each. There is no spark proper during the undisturbed 

 passage of these flames ; but if the latter be blown aside, a 

 stream of true bright-line sparks is seen passing between the 

 terminals. This torrent of flame (which, owing to the rapidity 

 with which the currents are delivered by the machine, is ap- 

 parently continuous) may be maintained for any length of 

 time. It would seem more than probable that this spark may 

 give some very valuable results in spectrum-analysis. The 

 sparks resemble those given by my great coil (described in 

 Phil. Mag. 1877, vol. iii. p. 30) with large battery-power and 

 with a mercury break ; but with that instrument it is doubtful 

 whether such thick sparks could be produced at short intervals 

 or in a rapid shower as in this case. 



In vacuum-tubes, exhausted so as to show bulbous stria?, 

 the effect is excellent. The stria? appear perfectly steady, as 

 with a battery like Mr. Gassiotfs or Mr. De la Hue's ; and 

 their brilliancy and configuration can be controlled by means 

 of a shunt in the secondary circuit, formed of a column of gly- 

 cerine and water, so as to diminish at will the amount of current 

 flowing towards the tube. 



But I postpone an account of various experiments made with 

 this method until a future occasion. 



I am, Gentlemen, 

 Sevenoaks, Yours faithfully, 



September 20, 1879. "\V. SpoTTISWOODE. 



2D2 



