36 Mr. Ritchie on a Torsion Galvanometer. 



inch, and then at the distance of one-eighth of an inch, the 

 power gained will only be as the square root of .25 to the 

 square root of .125, or nearly as 50 to 35 ; and the hydrogen 

 constantly escaping, and partially occupying the place of the 

 liquid in the narrow cell, considerably diminishes this apparent 

 increase of power. This circumstance ought not to escape the 

 attention of philosophical instrument-makers in the construc- 

 tion of batteries for electro-magnetic purposes. 



Considerable uncertainty still prevails with regard to the law 

 which connects the conducting powers of metallic wires with 

 their lengths. According to Professors Barlow and Gumming, 

 the law is the same as that established for fluid conductors. 



According to the experiments of M. Becquerel, the conduct- 

 ing powers of metallic wires are simply as their lengths. The 

 following experiment will set the question at rest. 



EXPERIMENT V. 



The galvanometer I have hitherto used requires the following 

 modification for this investigation. Form the rectangle of a 

 single copper wire, and suspend the magnetic needle directly 

 over it, and in the same direction. Take a certain length of 

 the same copper wire, and connect it with a small elementary 

 battery, turn the key, and observe the degree of torsion. Take 

 nine times the length of the same wire, and repeat the experi- 

 ment with the same battery and acid, and the number and 

 degrees of torsion will only be one-third of those obtained in 

 the first experiment. This experiment I repeated with dif- 

 ferent lengths of bell- wire, and always found that the intensity 

 of the current was inversely as the square roots of the lengths 

 the same as the law for liquid imperfect conductors. 



M. Becquerel seems to have fallen into the mistake we have 

 now pointed out, by using a galvanometer made of a long wire 

 formed into a coil, and neglecting the resistance the electric 

 current must have experienced in passing through the instru- 

 ment itself. 



The conducting powers of metallic wires, or their ribands, 

 for common electricity, depends almost entirely on their surface, 

 without any reference to their thickness. The fact would seem 

 to be, that common electricity glides along the surface of the 



