50 Mr. Forbes on eliciting an Electric Spark 



upon the multiplier and upon the frog, that a very powerful 

 and instantaneous current of electricity was conveyed through 

 the helix at the moment of making the contact of the connect- 

 ing iron with the magnet. 



In an early stage of my experiments I had, as far back as 

 the 30th of March, obtained a spark from the magnet, which, 

 however, being unable to repeat, from circimistances of which 

 I aftei-wards became aware, I did not choose to publish at the 

 time. I accordingly proceeded closely to investigate the cir- 

 cumstances under which sparks were to be obtained from 

 feeble galvanic currents of low intensity. I used the common 

 cylindrical electro-magnetic battery, in which, by varying the 

 charge of acid, I could obtain any required power. Thus 

 I adjusted it till I obtained from a momentary current nearly 

 the same action on the multiplier as 1 had developed by the 

 magnet. Removing it into a dark place, I found that sparks 

 Avere obtained at the instant of making and breaking the cir- 

 cuit connecting the cups of the battery. Satisfied that I had 

 a sufficient current of electricity, I proceeded to apply to the 

 magnet the conditions which I had found most effectual for 

 eliciting the spark. These were, 1st, That the spark is more 

 easily obtained at the instant of interrupting than that of com- 

 pleting the galvanic circuit: 2nd, That of the combinations 

 which I tried, a fine pointed iron-wire suddenly withdrawn 

 from contact with a surface of pure mercury, forming part of 

 the circuit, was the most regular in exciting the spark, and 

 that a good deal depended upon the suddenness of the inter- 

 ruption ; and, 3rd, That the spark was easiest obtained from 

 the mercury, not at the horizontal upper surface, but where 

 capillary action attracted it to the sides of the containing 

 vessel; and that this was independent of the material of the 

 vessel, being the same with wood, glass, and metal. 



I shall now briefly notice the arrangement of the apparatus 

 with which, on the 13th of April, I succeeded in obtaining 

 the spark at pleasure. 



The large natural magnet is represented at A. A cylin- 

 drical connecter of soft iron a b, passing through the axis of 

 the helix c, was made to connect the poles of the magnet; ac- 

 curacy of contact was found to be of considerable 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 assistant to 

 bring up the connecter rapidly and accurately to the magnet 

 in the dark. The helix c consisted of about 150 feet of 

 copper-wire, nearly one-twentieth of an Mich in dianuler, 7^ 



