Mr. Ritchie on a Torsion Galvanometer. 37 



metal, being prevented from escaping by the pressure of the am- 

 bient air, whereas Voltaic electricity requires a certain thickness 

 of metal for its transmission*. Voltaic electricity, from a single 

 pair of plates, seems to be conducted from molecule to molecule, 

 in some measure resembling the conduction of caloric. Hence, 

 if the diameter of the wire be too fine to allow of this depth of 

 metal, a considerable portion of the electric fluid will be stopped. 

 But, provided the wires be sufficiently thick to allow of this 

 necessary depth of the electric film, then the conducting power 

 ought to be nearly as the circumference of the wire, or as its 

 diameter. If one of the wires be very fine, and the other of a 

 large diameter, this law could not exist. This fact was clearly 

 proved by the following experiment. 



EXPERIMENT VI. 



Having taken equal lengths of very fine copper wire and of 

 common bell wire, I used them successively as conductors from 

 the same elementary battery, and ascertained the degrees of 

 torsion as in the former experiments, and found that the large 

 wire conducted better than in the mere ratio of the diameters. 

 For example, the diameter of the one wire was scarcely three 

 times that of the smaller, yet the ratio of their conducting 

 powers was nearly as one to four. I then passed the thick wire 

 through rollers, till it was reduced to a very thin riband, hav- 

 ing its external surface nearly twice that of the original wire, 

 but instead of conducting double the quantity of the original 

 wire, it conducted only three-fourths of that quantity f. 



From the law established in the fourth Experiment, we need 

 scarcely despair of seeing the Electro-Magnetic Telegraph 

 established for regular communication from one town to another, 

 at a great distance. With a small battery, consisting of two 

 plates an inch square, we can deflect finely-suspended needles 



* Hence if a metallic rod be raised to a red heat, its power of conducting com- 

 mon electricity is increased, whilst its conducting power for Voltaic electricity is con- 

 siderably diminished. 



f The fact here established bears a striking analogy to a curious fact discovered 

 by Mr. Barlow. He found that it requires a certain thickness of iron or steel to 

 receive the magnetic influence Is there any relation between the thickness of 

 the iron or steel necessary to receive the magnetic influence and the thickness of 

 the conductor necessary to convey that kind of electricity which acts most power- 

 fully on the needle ? " 



