and its Effects. 3 



meter of 1^ inch, and an external one of 2§ inches. Two of 

 these contain an extremely thin wire, 0*15 millim. in diameter, 

 and the third a considerably thicker one, 0'25 millim. in dia-- 

 meter. In each of the two first the wire is about 10,000 feet 

 long, and makes about 16,000 windings, in eight parts, each of 

 which contains 33 layers. The wire of the third coil is about 

 2400 feet long, and each of its parts contains 19 layers. For 

 certain purposes a fourth coil containing about 1200 feet of the 

 thicker wire was constructed ; its diameter was such as to permit 

 of its being inserted in one of the primary coils to be afterwards 

 described. 



Although the greatest possible care was taken in the prepara- 

 tion of all these coils, and although they are, perhaps, more 

 effective than others of equal dimensions constructed according 

 to the usual method, still the desired object is not yet perfectly 

 attained ; on the contrary, the conviction forced itself upon me- 

 that the insulation was far from sufficient to protect the coil from 

 the disturbing effects of internal sparks. 



This was particularly perceptible in the second of the coils 

 formed of fine wire. Immediately after its construction, the 

 phaenomena of sparks and tension which it exhibited were extra- 

 ordinary, indeed wonderful, but after a quarter of an hour its 

 action considerably decreased. This condition remained un- 

 changed for months, and for my purposes was satisfactoi'y 

 enough, for by employing two pairs of Grove's battery I could 

 obtain a spark at a distance of almost 12 millims. In all pro- 

 bability this condition would have been permanent had I not 

 afterwards had occasion to use four pairs of Grove's battery. 

 With this number I obtained spai'ks at a distance of 16 millims. 

 and upwards, but only for a few minutes ; afterwards the distance 

 decreased considerably, and at length, when two pairs were again 

 used, the distance, instead of being 12 millims. as before, was 

 only 9"6 millims. 



From this we may conclude, that in order to maintain an 

 induction coil in good condition, it must not be strained too 

 much, — at least when the current is broken so as to obtain 

 spai'ks in air of ordinary density, because in svich cases the elec- 

 tricity excited possesses great tension, and strives everywhere to 

 break open a path between the windings of the wire. Of course 

 such a deterioration of the coil need not be feared in experi- 

 ments where the poles of the induction coil are connected by a 

 pretty good conductor, for then the tension is either not so great, 

 or is not at all present. In such cases, too, the wire need not 

 be so perfectly insulated. For example, the ])h:cnomena of light 

 iu a partial vacuum are just as good when the wire is tolerably 

 well insulated, as when the insulation is as perfect as possible ;. 



B2 



