292 M. Matteucci on the Voltaic Arc. 



a black powder consisting of finely divided metal, and the 

 mark formed on the plate has no border. 



My principal object in making these researches being the 

 examination of the production of light at the negative pole, in 

 order not to have recourse to the hypothesis entertained on 

 this subject of the light being produced by one pole and the 

 heat by the other, I made a large number of experiments with 

 the positive and negative points of two different metals. I 

 found that the fixed light at the negative pole is never pro- 

 duced without the presence of a platina point at the positive 

 pole, and in this case the nature of the metal of the negative 

 point is indifferent; on the contrary, if the negative point is of 

 platina, and the positive pole is terminated by points of iron, 

 copper, zinc or silver, the fixed light no longer exists on the 

 negative point, or at all events the phaenomenon becomes much 

 confused. I was thus led to suppose that the phaenomenon, 

 being dependent on the nature of the metals, and produced 

 principally with platina, was owing to the positive pole be- 

 coming more heated, and to the particles detached from this 

 pole and transferred to the negative pole, becoming incandes- 

 cent from their very small size. It is evident that with metals 

 which readily oxidize and burn in the air, these phsenomena 

 are no longer produced in the same manner as with platina. 



There still remained to ascertain the cause of this unequal 

 heating of the substance of the two poles. The voltaic arc 

 which I studied is produced in a circuit which is sometimes 

 formed by the contact of the two points, and at other times 

 imperfectly established by the transference of matter with the 

 spaik. I passed an electric current produced by a constant 

 battery through two cylindrical rods of iron or lead, which 

 were in contact at their bases. Each of these rods had near 

 its base a very small hole, into which was inserted a thermo- 

 electrical clasp communicating with the galvanometer. I 

 was thus able to measure the temperature developed by the 

 passage of the current in the metallic rod near the place of 

 interruption, or, more exactly, near the ends of the two rods 

 in contact. With this arrangement I was easily able to con- 

 vince myself that the temperature developed by the passage of 

 the current was at its maximum near the place of interrup- 

 tion; and that in order to cause this temperature to vary, it 

 sufficed to alter the reciprocal pressure of the two rods. In 

 proportion as the pressure decreased, without any perceptible 

 alteration in the electric current, the temperature of the rods 

 constantly rose. Thus the thermo-electric current developed 

 by the clasp, being from ten to fifteen degrees when the two 

 rods were pressed hard one against the other, rose from sixty 



