32 Mr. W. Spottiswoode on a large Induction-coil. 



with an internal diameter of 3*75 inches and an external of 

 4*75 inches. The total weight of this wire is 55 lbs. 



The other primary, which is intended to be used with bat- 

 teries of greater surface, e. g. for the production of short thick 

 sparks, or for spectroscopic purposes, has a core of iron wires 

 •032 inch thick, forming a solid cylinder 44 inches long and 

 3*8125 in diameter. The weight of this core is 92 lbs. 

 The copper wire is similar to that in the primary first de- 

 scribed ; but it consists of 504 yards wound in double strand 

 forming three pairs of layers whose resistances are *181, *211, 

 '231 ohms respectively. Its length is 42 inches, its external 

 diameter 5 '5, and its internal 4 inches. Its weight is 84 lbs. 

 By a somewhat novel arrangement, these three layers may 

 be used either in series as a wire of '192 inch thickness, or 

 coupled together in threes as one of *576 inch thickness. 

 It should, however, be added that, owing to the enormous 

 strength of current which this is capable of carrying, and to 

 the highly insulated secondary coil being possibly overcharged 

 so as to fuse the wire, this larger primary is best adapted for 

 use with secondary condensers of large surface, for spectrum- 

 analysis, and for experiments with vacuum-tubes in which it 

 is desirable to produce a great volume of light of high inten- 

 sity as well as of long duration at a single discharge. The 

 alternate discharges and flaming sparks can also be best pro- 

 duced by this primary. It has been used for high-tension 

 sparks to 34 inches in air, the battery being 10 cells of Grove's 

 with platinum plates 6^ x 3 inches. Great facilities for the use 

 of different sets of batteries are afforded by the division of 

 this primary into three separate circuits, to be used together 

 or separately; and by a suitable arrangement of automatic 

 contact-breakers, the primary currents may be made to follow 

 in a certain order as to time, duration, and strength, with 

 effects which, when observed in the revolving mirror, will 

 doubtless lead to important results in the study of striae in 

 vacuum-tubes. 



We now come to the secondary, which consists of no less 

 than 280 miles of wire, forming a cylinder 37 '5 inches in 

 length, 20 inches in external, and 9*5 inches in internal diameter. 

 Its conductivity is 94 per cent. ; and its total resistance is equal 

 to 110200 ohms. The whole is wound in four sections, the 

 diameter of the wire used for the two central sections being 

 •0095 inch, and those of the two external being -0115 inch 

 and *0110 inch respectively. The object of the increased 

 thickness towards the extremities of the coil was to provide 

 for the accumulated charge which that portion of the wire 

 has to carry. 



