Force in the Voltaic Arc. 359 



fifteen Bunsen's elements. It is moreover to be remarked that the 

 numbers obtained are the means of the electromotive force. This 

 is doubtless strongest during the first moment after the extinction 

 of the luminous arc, and then diminishes in intensity till it quite 

 ceases. If the lever could be more quickly reversed than has 

 been heretofore possible, and thereby let the closing of the mag- 

 netometer circuit follow more rapidly the opening of the principal 

 circuit and the extinction of the luminous arc, a higher value 

 would be obtained for the electromotive force of the luminous arc. 

 4. The source of electricity in the disappearing luminous 

 arc cannot be of the nature of an electromotive force due to 

 contact. There are indeed two contacts between carbon and 

 gas; but these act in opposite directions and neutralize each 

 other. It is possible that when the experiments are made in air, 

 the moisture present is decomposed and produces a polarization 

 of the carbon points; but even if this were the case, it would 

 only explain an insignificant part of the electromotive force found. 

 Just as little can the force be an induction-phenomenon ; for the 

 conditions for the production of such a one do not exist in the lumi- 

 nous arc. It also seems to have nothing in common with the elec- 

 tromotive force discovered by Quincke to accompany the passage of 

 liquids through porous diaphragms. The only supposition which 

 might possibly be made is that the force is of thermoelectric 

 origin. The positive pole is considerably more heated than the 

 negative; and we have thus in the luminous arc two contacts 

 between carbon and gas, of which one has a considerably higher 

 temperature than the other. But it appears improbable that this 

 thermoelectric force could have obtained so considerable a value 

 as that we have observed*. In order to ascertain with certainty 

 he real state of the case, the following experiments were made. 

 If the force in question is of thermoelectric origin, and caused 

 by the circumstance that the positive pole has a higher tempera- 

 ture than the negative one, the current observed must disappear 

 when the temperature of the negative pole is raised to the same 



* Mr. Wilde, in Pogg. Ann. vol. cxi., has communicated a paper in which 

 it is demonstrated that a delicate magnetometer gives a considerable 

 deflection if it is connected with a battery of twenty Bunsen's elements as 

 in fig. 1. Mr. Wilde does not mention any precautions which he used to 

 convince himself that in his experiments there was no simultaneous closing 

 on opening the principal current and introducing the magnetometer circuit. 

 If there is a simultaneous closing, deflections of the galvanometer are 

 always obtained ; but these are then caused by the principal current itself. 

 Mr. Wilde has made no experiments with the view of measuring the strength 

 of the current or the intensity of its electromotive force. Hence Mr. Wilde 

 did not hesitate to assume that the deflections obtained were caused by a 

 thermoelectric current which resulted from the contact of the pole points 

 (heated to unequal temperatures) with the interjacent gas. 



