206 On the Induction Apparatus and its Effects. 



irou is employed. "\^lieu the soft iron core is inserted in the 

 coil, the intensity of both increases, but it is the spark which 

 especially gains magnitude and brilliancy, and in these respects 

 is at least three times greater than the spark in free air. It 

 actually assumes the form of a small flame bursting forth on 

 all sides from under the pin, in the midst of which the point of 

 the pin may be distinguished as a -brilliant luminoiis point. The 

 blue light, on the contrary, covers all the neighbouring parts of 

 the pin, which forms the negative pole, with its beautiful bright- 

 ness, and would undoubtedly spread itself more were the hammer 

 suitably constructed*. 



When employing soft iron, the condenser has scarcely any 

 influence upon the phsenomenon ; without soft iron it weakens 

 the spark a little, and destroys the blue light completely. 



The primary wires being connected end to end, the spark 

 becomes smaller and less luminous. The blue light, on the con- 

 trary, is more developed, if not more beautifully blue. 



When the cun-ent is reversed, it is the vibrating hammer 

 which becomes covered with the blue light; this is not only 

 beautiful to observe, but also gives rise to a remarkable occur- 

 rence : the anterior part of the tongue between the pin and the 

 armature, which is not traversed by the current, receives a lumi- 

 nous covering. 



The above-mentioned phrenomena are well calculated to render 

 perceptible the influence of the soft iron core in its several forms. 

 A massive soft iron bar introduced into the primary coil weakens 

 the action which this coil exerts when alone, in exactly the same 

 manner as it decreases the spark when the hammer vibrates in 

 free air. On the contrary, a hollow bundle of wires, whose walls 

 are tolerably thick, is just as eff'ectual as a full bundle, and its 

 action is little or not at all diminished when a solid bar is intro- 

 duced into the hollow axis. The most efi"ectual means of in- 

 creasing the luminous phfenomena, however, is to place the coil 

 in the middle of a very long bundle of soft iron wires ; for this 

 purpose I employed the one before described fj, which is three 

 times as long as the coil itself. 



The insertion of the induction coil into the primary coil has 

 no influence upon any of the phsenomena here described, so long 

 as the former remains unclosed, and does not present a closed 

 circuit in its interior. When closed, however, either by metallic 

 contact or by a well-exhausted electric egg, its insertion weakens 

 the luminous phaenomena at the hammer, vibrating in a vacuum 



* Neef himself allowed the hammer to vibrate in a vacuum (Pog. Ann. 

 vol. Ixvi. p. ■432), but the small intensity of his apparatus difl not, of course, 

 permit him to see the phsenomenon in its full development. 



t Phil. Mag. vol. x. y). fi. 



