92 Mr. H. Wilde^s Experimental Researches 



powerful electromagnet (67, 68), a bright spark was in like 

 manner obtained twenty-five seconds after all connexion with 

 the magneto-electric machine had been broken*. 



39. None of the effects described, such as the great sustain- 

 ing-power of the electromagnet above that of the permanent 

 magnets (34), the increased brilliancy of the spark at the point 

 of disjunction of the wires (37), or the spark from the electro- 

 helices after all connexion with the magneto-electric machine had 

 been broken (38), were observed when the alternating current 

 from the machine (16) was transmitted through the electrohelices 

 instead of the direct current from the commutator (15). Under 

 these conditions it was found that neither the small electromag- 

 net (22, 23) nor the large electromagnet (33) would sustain 

 even a weight of 1 lb. 



40. It was at first thought that the great difference observed 

 between the sustaining-power of the electromagnet and that of the 

 permanent magnets which excited it might be due to the helices 

 surrounding the electromagnet absorbing or retaining the elec- 

 tricity transmitted through them in a static condition, in the 

 manner observed in insulated submarine and subterranean tele- 

 graph wires, an investigation of which phenomenon, as it was 

 manifested in gutta-percha-covered wires submerged in a canal, 

 and in similar wires laid underground between London and Man- 

 chester, was made by Faraday in 1853 f. 



41. Eor the purpose of ascertaining whether this view of the 

 subject was correct, a very small and delicate electromagnetic 

 balance was constructed, similar in principle to the one shown in 

 fig. 8. The small electromagnet, fixed beneath one end of the 

 balance, was coiled with a strand of insulated copper wires 6 feet 

 in length and 0*15 of an inch in diameter. The submagnet was 

 prevented from coining into contact with the poles of the elec 

 tromagnet by means of regulating-screws. Other regulating- 

 screws limited the movements of the balance in the opposite di- 

 rection ; and the distance of the submagnet from the electro- 

 magnet could also be adjusted, by means of the same regulating- 

 screws, to suit the different quantities of electricity transmitted 

 through the electrohelices. 



42. This electromagnetic balance was placed in the middle of 

 the circuit of the electromagnet (33), i. e. at the point where the 

 two electrohelices were joined together. The poles of the mag- 

 neto-electric machine were then connected with the free extremi- 



* Since this paper was read I have discovered that Dr. Page, in 1851, 

 succeeded in obtaining a spark from an electromagnet, coiled with a very 

 long wire, half a second after all connexion with the battery had been 

 broken. — Silliman's American Journal of Science, vol. xi. p. 88. 



t Proceedings of the Royal Institution, January 20, 1854. 



