REGENT PEOGKESS IN PHYSICS. 431 



Neef has shown that this appearance of light, when occurring 

 simply without the action of secondarj^ currents, is neither a common 

 electric spark, viz., a spark j)a.ssing from one pole to the other, nor 

 can it be attributed to a combustion of metal. — (P. A. LXVI, 414.) 



In his so called magnet electrometer, as described in the 3d edition 

 of my ^' Lehi'buch der PJiysik," vol. II, p. 251, a continual closing 

 and breaking of the current takes place at c which is accompanied by 

 a corresponding ap^^earance of the light. But in this form of the 

 apparatus the observation of the phenomenon is rendered, difficult by 

 the width of the hammer which strikes the platinum plate. In a very 

 convenient modification of this apparatus by Desaga_, in Heidelberg, 

 which will be described hereaiter, a platinum point is substituted for 

 the hammer, so that the contact is made and broken in rapid alterna- 

 tion between a platinum point and plate. 



When the apparatus is in action, light is observed, at the place of 

 separation, which, on account of the rapidity of the oscillation, seems 

 to be contiDuous. To the naked eye, when protected from the day- 

 light, the light appears violet, whether the positive current pass from 

 the point to the plate or inversely ; but the point of light is so ex- 

 tremely small that Neef was induced to examine the phenom.enon with 

 the microscope. By the aid of this instrument he found that the light 

 always a,ppears at the negative pole only. 



The;^e observations are best made with a microscope which magnifies 

 25 to 50 times, which permits the objective to be removed to at least 1^ 

 inch from the point of light. 



When the positive current passes from the plate to the point, the 

 latter appears enveloped in violet light, while the plate remains en- 

 tirely dark. 



At the lowest extremity of the point within the continuous violet 

 light there appear single, extremely fine, dots of dazzling white light 

 with a sort of swarming motion ; and tov/ards the upper extremity of 

 the violet envelope there is also seen light of the same color but of 

 much greater intensity, flashing out, and yet no passage to the other 

 pole could be observed. The last mentioned flashing gradually dis- 

 appears as the force of the current decreases. 



When the current passes in the opposite direction, and the point, con- 

 sequently, is positive, it remains quite dark and the violet light is 

 spread upon the plate around the point of nearest approach. In this 

 case, too, there is no spark, strictly so called, to be observed, and the 

 uniform violet glimmering light can scarcely be attributed to a com- 

 bustion of the platinum. Besides, Neef also especially remarks that 

 this phenomenon of light takes place at the negative pole only, while 

 the greatest heat is always developed at the positive pole. 



