HISTORY OF SCIENCE. 



niences caused by sparks at the point where the interruption takes place. 

 In the smaller Ruhmkorff coils a much simpler arrangement for break- 

 ing and making contact suffices, and no auxiliary battery is needed. At 

 L there is a little arrangement for reversing the direction of the battery 

 current. Its action will be readily understood by an inspection of the 

 actual apparatus, but a*description would be tedious. The end of the 

 outer coil terminates at the brass caps A, B, mounted on glass pillars 

 and provided with screws for attaching wires. The induction coils 

 first made did not give a spark longer than -^th of an inch ; but when 

 M. Fizeau attached a condenser to the inducing coil, sparks of much 

 greater length were attainable. The induction coil is therefore now 

 always constructed with a condenser, which is commonly contained 

 in the hollow base of the instrument. The condenser is formed by 

 pasting a sheet of tinfoil on each side of a piece of varnished silk or 

 paper of considerable area. The compound sheet is folded up or 

 divided into pieces which pack into the base of the instrument, and 

 the tinfoil on one side is in communication with one pole of the battery 

 coil, while that on the other side is connected with the other pole. 

 The large inductive coils will give between the poles a constant stream 

 of sparks several inches in length, attended by a loud cracking noise. 

 There is a word of explanation required as regards the direction oi p 

 the induced current. We have seen that an inverse induced current 

 should occur when the battery current is established, and a direct 

 current when it is broken. Now, for certain reasons which it would 

 occupy too much space to explain here, the direct induced current has 

 much greater energy than the inverse one ; so that if the induced 

 circuit be broken at any point, the sparks which appear there are 

 due to the direct current only, the inverse current not having the power 

 to leap across the interval. 



The induction coil has been a boon to science. It supplies, without 

 trouble or attention, a continuous stream of electricity of high tension, 

 like that of the common machine, but far greater in quantity. By its 

 aid we obtain the intensely ignited metallic vapours (for sparks are 

 nothing else), which yield their characteristic lines in the spectroscope, 

 and bypassing the discharge through a nearly exhausted tube, we have 

 the means of producing the spectrum of the residual gas. To give 

 the spectra of gases is not the only property of the Geissler tube 

 traversed by the induction current. There are, indeed, no more 

 beautiful effects presented to the eye in any scientific experiment than 

 those of the illuminated Geissler tubes. In these tubes the ignited 

 gas assumes, under certain circumstances, a curious stratified appear- 

 ance, of which the annexed Fig. 298 will give some notion. No very 

 satisfactory explanation of this appearance has yet been advanced. 

 It is highly probable that a complete examination of the many curious 

 phenomena presented by Geissler's tubes may yet open to our view 

 some of Nature's deepest secrets. 



