ELECTRICITY. 



45 



to be that of controlling the tendency of 

 the electric fluid to diverge, and of con- 

 centrating the whole power of the charge 

 into a single point. 



( 1 64.) This repulsive tendency is also 

 well illustrated by the following experi- 

 ments made by Dr. Priestley. If a clean 

 brass chain, previously dipped in melted 

 resin, be laid upon paper, and the 

 charge of a battery of at least 32 square 

 feet "be sent through it, the resinous 

 coating will be thrown off from every 

 part of the chain, which will be left per- 

 fectly clean, and free from resin. If a 

 brass chain be laid upon a piece of 

 glass, and a similar charge passed 

 through :it, the glass will be marked in 

 a beautiful manner on every part of its 

 surface, where it had been touched with 

 the chain, every spot having the width 

 and colour of the link. The metal may 

 be scraped off the glass at the outside of 

 the marks, but in the middle part it is 

 forced within the pores of the glass. 

 Dr. Priestley communicated a similar 

 tinge to glass, by means of a silver 

 chain, and small pieces of other metals ; 

 but he could not succeed with large 

 pieces. 



(165.) The effects of accumulated 

 electricity upon metallic bodies, are re- 

 ferable, for the most part, to the agency 

 of the heat produced by its passage 

 through them ; yet the phenomena, in 

 many cases, indicate also the operation 

 of other forces. By the transmission, 

 through a piece of metal, of repeated 

 shocks, which are not powerful enough 

 to effect its fusion, or even ignition, a 

 permanent alteration may be produced 

 in its form, such as would not have re- 

 sulted from heat alone. Dr. Priestley 

 and Mr. Nairne found by experiment, 

 that a chain through which an electrical 

 charge had passed, undergoes a diminu- 

 tion in its length. A piece of hard 

 drawn iron wire, ten inches long and 

 one hundredth of an inch in diameter, 

 was found, after fifteen discharges, to 

 have lost one inch and one tenth of its 

 length ; and the increase of thickness 

 seemed to be in proportion to this lon- 

 gitudinal contraction, for the wire had 

 not perceptibly lost any of its weight 

 during the experiment. A copper wire 

 plated with silver, of the same dimen- 

 sions as the former, underwent, by the 

 same treatment, a diminution of length 

 two thirds as great as that of the iron 

 wire. 



On the other hand, if the shocks 

 be transmitted through a wire which 



has a weight suspended by it, so as 

 to irive it considerable tension, the length 

 of the wire becomes increased instead 

 of diminished, as in the above experi- 

 ment. This is evidently owing to the 

 influence of the heat which accompa- 

 nies the passage of the electricity, and 

 which diminishes the cohesion of the 

 particles of the metal, and disposes 

 them to yield to the extending force 

 which the weight supplies. 



$ 2. Evolution of Heat by Electricity. 



(166.) The ignition and fusion of metals 

 by the electric discharge, are phenomena 

 which have been long observed. Thus 

 by passing a strong charge through 

 slender iron wires, they are ignited, and 

 partly melted into globules. It was 

 formerly believed that very large bat- 

 teries were necessary for obtaining this 

 effect; but if the wire be sufficiently 

 fine, the electricity accumulated in a 

 single jar of moderate size will suffice 

 for its production. The best material 

 for exhibiting this effect, is the finest 

 flatted steel sold at the watchmakers' 

 tool shops, under the name of watch 

 pendulum wire. Van Marum has given 

 a statement of the lengths of wires of dif- 

 ferent diameters, and of different metals, 

 which his powerful machine enabled 

 him to melt; when they were drawn 

 to the thirty-second part of an inch in 

 diameter, he found that he could fuse 

 120 inches of lead wire, and the same 

 quantity of tin wire ; five inches of iron 

 wire; three inches and a half of gold 

 wire ; and only one quarter of an inch 

 of wires of silver, copper, or brass. 



(167.) From the experiments of 

 Brooke and of Cuthbertson, it has been 

 inferred that the length of wire which is 

 thus melted by the electric discharge, 

 varies as the square of the quantity of 

 accumulated electricity which is sent 

 through it ; thus a combination of two 

 jars, charged to an equal degree, will 

 melt four times the length of wire which 

 one jar will melt. 



(168.) While the electric battery thus 

 effects the fusion, and even in some 

 cases the volatilization of metals, the 

 phenomena appear also to indicate the 

 action of propelling or dispersive forces, 

 as if the agent concerned in their pro- 

 duction was endowed with great mecha- 

 nical momentum. Thus the densest 

 metals are rent and dispersed with vio- 

 lence by the passage of accumulated 

 electricity If a slip of gold or silver 

 leaf be placed on white paper, and a. 



