1882.] on Matter and Magneto-Electric Action. 77 



macliine. You will notice that with the battery we can obtain either 

 long, bright, and thin sparks, or short and comparatively thick 

 discharges ; but, unless the latter are made very short, they occur 

 only at comparatively long and even perceptible intervals of time. 

 On the other hand, with the alternate machine, although the method 

 does not lend itself so readily to the production of long and bright 

 sparks, we can produce a perfect torrent of discharges more rapid and 

 more voluminous than by any other means yet devised. Long bright 

 sparks can, however, be obtained by interrupting the flow of the 

 currents from the machine, and by allowing only single currents to 

 pass at comparatively long intervals. It may be interesting to know 

 that the number of currents given out by the machine, and consequently 

 the number of discharges issuing from the coil, is no less than 35,200, 

 that is, ITjGOO in each direction, per minute. The number may be 

 determined by the pitch of the note which always accompanies the 

 action of an alternate machine. 



A comparison of the two methods may also be made when a 

 Leyden jar is used as a secondary condenser. This application of the 

 jar is well known as a valuable aid in spectroscopic research ; and the 

 employment of the alternating machine so materially heightens the 

 effects that, judging from some experiments made in the presence of 

 Mr. Lockyer, and from others of a different character in the presence 

 of Professor Dewar, I am led to hope from it a further extension of 

 our knowledge in this direction. In order that you may form, at all 

 events, some rough idea of the nature of such discharges, I venture, at 

 the risk of causing some temporary inconvenience from the noise, to 

 project the spectrum of this sj)ark. 



I will detain you with only one more instance of comparison. 

 The ordinary effect of an induction coil in illuminating vacuum tubes 

 is well known. The result is usually rather unsteady. Several 

 instruments have been devised to obviate this inconvenience, e. g. the 

 rapid breakers described in the ' Proceedings ' of the Eoyal Society 

 (vol. xxiii. p. 455, and vol. xxv. p. 547), or the break called the " Trem- 

 bleur" of Marcel Deprez (see 'Comptes Eendus,' 1881, I. Semestre, 

 p. 1283). The use of the alternating machine, however, not only gives 

 all the regularity in period, and uniformity in current, aimed at in 

 these instruments, but also at the same time supplies currents of great 

 strength. The result is a discharge of great brilliancy and steadiness, 

 and it is perhaps not too much to say that the effects are comparable 

 to those obtained with Mr. De La Rue's great chloride of silver 

 battery. The configuration of the discharge produced in this way 

 can also be controlled by a suitable shunt applied to the secondary 

 circuit ; for example, one formed by a column of glycerine and water, 

 or the one consisting of a film of plumbago spread upon a slab of 

 slate, constructed by my assistant Mr. P. Ward, and here exhibited. 



One test of the strength of current passing through a tube is 

 the amount of surface of negative terminal which it will illuminate 

 with a bright glow. I here have a tube with terminals in the form 



