ELECTRICITY. 507 



There are many beautiful experiments designed to ex- 

 hibit in a striking way the illuminating power of elec- 

 tricity. Some of these we shall describe. 



The spiral tube. This consists of a common glass 

 tube with little circular pieces of tin foil pasted upon it, 

 so as to be as near each other as possible without touch- 

 ing. These pieces are arranged in such a manner that 

 they pass round the tube in a spiral form, one end of 

 which is to be connected with the prime conductor. 

 On turning the machine the conductor becomes charg- 

 ed, and the fluid passes off through these discs of tinfoil, 

 producing a spark at each interruption ; and as the 

 passage is instantaneous, the appearance is that of a 

 beautiful spiral line of light. The operator sometimes 

 endeavors to perform this experiment by means of the 

 Leyden jar. It is, however, a remarkable fact which 

 we do not recollect to have seen noticed in any treatise, 

 that very many of the experiments with electric light, 

 succeed altogether better with the conductor than with 

 the jar. The conductor gives a longer spark, and the 

 charge from it forces its way through a much longer se- 

 ries of interruptions. 



The spiral tube prepared as above described, is often 

 inclosed in another tube a little larger, so that the tin- 

 foil is protected from injury. Sometimes, instead of a 

 tube, a flat plate of glass is used, and the circles of tin- 

 foil or of nheet lead are pasted upon it in any way that 

 fancy may direct. The interruptions are sometimes so 

 arranged that the letters of a word are pictured in light, 

 or a profile, or a rude sketch of any kind. Some- 

 times a long and narrow plate of glass has a serpentine 

 row of discs of tin foil upon one side, while th other is 

 painted with transparent colors, which gives to the light 

 a variety of hues. 



The luminous jar. A Leyden jar is coated with de- 

 tached pieces of tin foil coming almost into contact with 

 each other, instead of having the ordinary continuous 

 covering. While this is charging, sparks will be seen 

 darting from one piece of the tinfoil to another, illumi- 

 nating the whole surface in a beautiful manner. 



The illumi'iaied thumb. If two wires lying upon the 



