174 Dr. Tyudall on the Progress of the Physical Sciences : 



Rejecting the method of inferring the strength of a current 

 from its effect in the magnetization of a steel needle, the author 

 makes the heating of a fine platinum wire, introduced into the 

 circuit, and passing through an air-thermometer of his own con- 

 struction, the measure of the strength. The following short 

 paper, which has been translated for some time, and held back 

 with the view of presenting at once an abstract of the whole of 

 this important investigation to the readers of the Philosophical 

 Magazine, establishes the fact, that not only does the primary 

 current affect a second wire placed near it, but that the various 

 portions of the said current affect each other ; the strength of 

 the current being thus proved to depend in some measure upon 

 the shape of the wire through which it passes. 



It is known, writes the author, that the current of the electric 

 battery acts inductively upon the mass of the connecting wire. If 

 a second conducting wire be connected with two points of the 

 original circuit, the action of the induced current maybe exhibited, 

 partly in a direct manner, and partly, as I have already shown, by 

 the disturbances which the laws of the branch current experience. 

 From this, however, it does not follow that in the simple con- 

 necting wire itself the induced current will exhibit any sensible 

 action ; for in this case, as it has no circle to move in, its effect 

 must be very small in comparison with that of the original cur- 

 rent. The question, whether by an approximation of two por- 

 tions of the connecting wire an alteration of the current of 

 discharge takes place, must be referred to experimental decision. 

 This experiment was made by me long ago with 26 feet of wire 

 wound into two plane spirals which were brought near each 

 other, the heating of the wire in the remaining portion of the 

 circuit being at the same time observed. There was no differ- 

 ence exhibited from which any inference could be drawn. Similar 

 experiments were made afterwards by Hankel with 317 feet of 

 wire, which was wound into two cylindrical spirals. To test the 

 current, however, Hankel chose, not the heating of the wire, but 

 the magnetization of steel needles which lay near it, and which, 

 by successive charges of the battery, were magnetized; the 

 magnetization was found different according to the manner in 

 which the spirals were united. We have here gained a parti- 

 cular fact of considerable interest, but the general question raised 

 by me remains still unanswered. The question was, when two 

 portions of the current are in a certain manner brought close 

 together, is this act accompanied by an increase of the tempe- 

 rature of the wire, or by a decrease thereof, or does the tempera- 

 ture remain unchanged ? The experiments on magnetizing de- 

 cide none of these questions ; and I found myself, therefore, com- 



