u 



76 Mr, Spottiswoode [March 31, 



drivea by an Otto gas engine. Tlie speed of the de Meritens' 

 machine, so driven, is about 1100 revolutions per minute. 



In this arrangement the currents in the secondary are of course 

 alternately in one direction and in the other, and equal in strength ; 

 so that the discharge appears to the eye, during the working of the 

 machine, to be the same at both terminals. 



The currents in the primary are also alternately in one direc- 

 tion and in the other, and consequently, at each alternation, their 

 value passes through zero. But they differ from those delivered 

 in the primary coil with a direct current and contact breaker in an 

 imjiortant particular, namely, that while the latter, at breaking, fall 

 suddenly from their full strength to zero, and then recommence with 

 equal suddenness, the former undergo a gradual although very rapid 

 change from a maximum in one direction through zero to a maximum 



in the opposite direction. The ordinary currents 

 with a contact breaker would be represented by a 

 figure of this kind, while those from the alternate 

 machine approximately by a curve of the following form. The 



rise and fall of the latter are, however, sufficiently 

 I 11 11 rapid to induce currents of high tension and of 



great quantity in tlie secondary. 



From these considerations it follows : first, that as the machine 

 effects its own variations in the primary current, no contact breaker is 

 necessary ; secondly, that as there is no sudden rupture of current, 

 there is no tendency in the extra current to produce a spark or any 

 of the inconveniences due to an abrupt opening of the circuit, and 

 consequently that the condenser may be dispensed with ; thirdly, that 

 the variations in the primary, and consequently the strength and 

 period of delivery of the secondary currents are perfectly regular ; 

 fourthly, that the strength of the currents in the secondary is very 

 great. With a 26-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 in length, an ordinary 4-inch coil is sufficient. 



Owing to the double currents, the appearance of the discharge is 

 that 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. This torrent of flame (which, owing to the 

 rapidity with which the currents are delivered by the machine, is 

 apparently continuous) may be maintained for any length of time. 

 The sparks resemble those given by my great coil (exhibited in this 

 theatre on Friday, April 13th, 1877, and described in the 'Philo- 

 sophical Magazine,' 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 order to contrast the effects of the two methods, I will excite 

 the coil, first with a battery, and secondly with the alternating 



