from a Natural Magnet. 201 



of the magnet; accuracy of contact was found to be of consider- 

 able importance to the success of the experiment, and one side 

 of the cylinder was carefully formed to a curve of about two 

 inches radius for this purpose. I found great advantage from a 

 mechanical guide, not represented in the figure, to enable an as- 

 sistant to bring up the connector rapidly and accurately to the 

 magnet in the dark. The helix c consisted of about 1 50 feet of 

 copper-wire, nearly one-twentieth of an inch in diameter, 7^ 

 inches long, and containing four layers in thickness, which were 

 carefully separated by insulating partitions of cloth and sealing- 

 wax. The one termination de of the wire, passed into the bot- 

 tom of a glass tube A, half filled with mercury, in which the wire 

 terminated, and the purity of the mercurial surface is of great 

 consequence to the experiment. The other extremity /' of the 

 helical wire communicated by means of the cup of mercury i, 

 with the iron-wire g, the fine point of which may be brought by 

 the hand into contact with the surface of the mercury in h, and 

 separated from it at the instant when the contact of the connec- 

 tor a b with the poles of the magnet is effected. The spark is 

 produced in the tube h. 



The success of the experiment clearly depends on the syn- 

 chronism of the production of the momentary current by con- 

 necting the magnetic poles, and the interruption of the galvanic 

 circuit at the surface of the mercury. This might be pretty 

 nearly ensured by a variety of simple mechanical contrivances 

 which suggest themselves, but as these would require very con- 

 siderable nicety in their execution, I have been satisfied with the 

 precision which may be insured by a good ear and an accurate 

 assistant, as I have thus, with a little practice, been able to 

 produce, for many times in succession, at least two sparks from 

 every three successive contacts. 



These sparks have generally a fine green colour ; that I ob- 

 tained on the 30th of March was in every respect similar to those 



