144 Prof. Van Breda on the Repulsion of a 



to each other. The unequal contraction of the two parts of the 

 conductor, of different form and often of two different metals, 

 sometimes causes a spontaneous rupture of this junction after a 

 longer or shorter time, as you have observed. 



There is fortunately one means of preventing this welding ; and 

 that is by using a metal, mercury, which is liquid to begin with, 

 and then you can observe the Amperian repulsion. In the same 

 article of Cosmos, of which some lines have been cited, we have 

 described an experiment which has much analogy with your own. 

 We described it thus : — 



"On one of the arms of a small and delicate balance is suspended 

 a copper- wire conductor about 3 millims. in diameter, and bent 

 in the form of an inverted U. The other arm is loaded with 

 weights which almost counterbalance the conductor ; that being 

 done, the two ends of the latter are respectively immersed in two 

 mercury cups, placed at such a height that they are not immersed 

 to a greater depth than 2 millimetres at most. As soon as the 

 circuit is closed so that a pretty strong current passes from the 

 mercury to the conductor, and from the other end of the con- 

 ductor to the mercury in the other cup, the conductor is briskly 

 repelled out of the mercury ; the current is broken, and the con- 

 ductor falls again to be again repelled, and so on indefinitely*.'" 



The current for this experiment ought to proceed from four 

 Grove's elements, each presenting an active surface of 12 square 

 inches, and connected so as to form a series of two elements of 

 double the size. By using a weaker current, the repulsion is not 

 strong enough to throw the conductor entirely out of the mer- 

 cury ; it merely rises a little as soon as the current begins to 

 pass. We have lately improved the apparatus by fixing at the 

 other end of the balance a small glass disc, on which there is a 

 scale divided into tenths of a millimetre. This can be observed by 

 means of a small fixed microscope magnifying about twenty times, 

 and with cross-wires on its ocular. In addition to this, plati- 

 num wires are soldered to each of the ends of the bent conductor ; 

 they are about a millimetre in diameter, and are covered with 

 glass so as only to leave the two ends free. The results obtained 

 by means of the apparatus thus modified have appeared interest- 

 ing to us, because they have confirmed us in our point of view, 

 which, in the explanation of all the motions cited, as well as in that 

 of the phenomena observed with hollow spheres by Mr. Gore, 

 and in Mr. Page's experiment with the Trevelyan instrument, 

 only attributes a secondary part to the heat developed by the 

 current ; it refers some exclusively, and others principally, to the 

 Amperian, just as you have hitherto done. In the case of Mr. 



* We have since learnt that this experiment had been previously made by 

 Mr. Faraday. 



