290 M. Matteucci 07i the Voltaic Arc. 



employed in the application of electricity for medical purposes. 

 With this instrument, which acts for several days with a few 

 of Bunsen's or Grove's cells, there is between a point and a 

 slip of platina a continual series of electric sparks correspond- 

 ing to the very close interruptions of the circuit. On obser- 

 ving the phaenomenon with the naked eye, the arc of light 

 would be thought to be continuous; but by looking upon a disc 

 the surface of which is painted with black and bright rays, 

 rotating with a certain velocity, it is eas}' to be convinced of 

 the discontinuity of this electric light. 



In all my experiments I employed two similar points of 

 platina, or of another metal, instead of a point and the plate 

 as extremities between which the spark should be emitted. 



I first examined the temperature of the two metallic points 

 at the moment the electric arc was produced ; and for this 

 purpose made very near to the extremities of the points a hole 

 which was scarcely one millimetre in diameter, and into which 

 was inserted the point of a thermo-electrical clasp of iron and 

 copper in communication with the galvanometer. When the 

 experiment is carefully performed, so as to have a continuous 

 series of sparks accompanied by a constant sound, there is also 

 a fixed deflection of the galvanometer. I have in this manner 

 proved and measured the difference of temperature of the 

 positive point compared with that of the negative point, the 

 latter being always lower. The difference varies with the 

 metals, as was to be expected ; I found it greatest with iron 

 and copper, iron and platina, and less with lead, bismuth and 

 zinc. 



I then studied the luminous phasnomena of this electric arc, 

 which in this case was of importance, as Dr. Neef of Frankfort 

 had never observed any light except at the negative pole. 

 According to that author, with a very weak current there is 

 constantly and only at the negative pole an electric light, 

 which he calls ■primary^ from its being, in his opinion, inde- 

 pendent of the presence of the matter of the poles. 



I made a long investigation of the voltaic arc obtained with 

 the electro-magnetic machine, observing it with the aid of 

 a microscope which magnified from 40 to 60 times. The 

 experiment, which is very beautiful and important, succeeds 

 best on employing iron or platina points and a very feeble 

 current. The following are the phsenomena which I constantly 

 observed : — 



1. The positive extremity is distinctly seen only in the state 

 of incandescence ; globules of red molten matter roll Over its 

 surface, which separate from it, leaving cavities, and are pro- 

 jected on to the negativepoint, where they form mushroom-like 



