Mr. AY. Spottiswoode on a large Induction-coil. 33 



Each of these sections was wound in flat disks ; and the 

 average number of layers in each disk is about 200, varying, 

 however, with the different sizes of wire, &c. The total 

 number of turns in the secondary is 341,850. 



The great length of the wire necessary can be easily under- 

 stood from the fact that near the exterior diameter of the coil a 

 single turn exceeds 5 ft. in length. The spark, it is believed, 

 is due to the number of turns of wire, rather than to its 

 length, suitable insulation being preserved throughout the 

 entire length. In order to ensure success, the layers were 

 carefully tested separately and then in sets, and the results 

 noted for comparison. In this way it was hoped that step by 

 step safe progress would be made. As an extreme test, as 

 many as 70 cells of Grove's have been used, with no damage 

 whatever to the insulation. 



The condenser required for this coil proves to be much 

 smaller than might at first have been expected. After a 

 variety of experiments, it appeared that the most suitable size 

 is that usually employed, by the same maker, with a 10-inch- 

 spark coil — viz. 126 sheets of tinfoil 18 x 8*25 inches in sur- 

 face, separated by two thicknesses of varnished paper, the two 

 thicknesses measuring *011 inch. The whole contains 252 

 sheets of paper 19 x 9 inches in surface. I hope, at some 

 future opportunity, to make further experiments with other 

 condensers. 



Using the smaller primary, this coil gave, with 5 quart 

 cells of Grove, a spark of 28 inches, with 10 similar cells one 

 of 35 inches, and with 30 such cells one of 37*5 inches and sub- 

 sequently one of 42 inches. As these sparks were obtained 

 without difficulty, it appears not improbable that, if the insu- 

 lation of the ends of the secondary were carried further than 

 at present, a still longer spark might be obtained. But special 

 adaptations would be required for such an experiment, the 

 spark of 42 inches already so much exceeding the length of 

 the secondary coil. 



When the discharging points are placed about an inch 

 apart, a flowing discharge is obtained both at making and at 

 breaking the primary circuit. The sound which accompanies 

 this discharge implies that it is intermittent, the time- and 

 current-spaces of which have not as yet been determined. 



With a 28-inch spark, produced by 5 quart cells, a block 

 of flint glass 3 inches in thickness was in some instances 

 pierced, in others both pierced and fractured, the fractured 

 pieces being invariably flint glass. If we may estimate from 

 this result, the 42-inch spark would be capable of piercing a 

 block 6 inches in thickness. 



Phil. Mag. S. 5. Vol. 3. No. 15. Jan. 1877. D 



