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



4. The velocity of propagation of ignition may be easily 

 determined for solid and liquid combustible bodies ; and the 

 numbers so obtained may be regarded as comparative quanti- 

 tative expressions for the liability to ignition of these sub- 

 stances. 



II. Description of a Large Induction-coil. 

 By William Spottiswoode, F.R.S. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



ALTHOUGH I have not as yet many experimental results 

 sufficiently complete for communication to your Maga- 

 zine, I still think that the construction of an induction-coil 

 capable of giving a spark 42 inches in length is an instrumental 

 feat deserving of record in the annals of science. I therefore 

 venture to submit the particulars of this coil, recently com- 

 pleted for me by Mr. Apps, of 433 Strand, to whose skill and 

 perseverance the success of the undertaking is due. 



The general appearance of the instrument is represented in 

 the following figure, by which it is seen that the coil is sup- 

 ported by two massive pillars of wood sheathed with gutta- 

 percha, and filled in towards their upper extremities with 

 paraffine wax. Besides these two main supports, a third, 

 capable of being raised or lowered by means of a screw, is 

 placed in the centre, in order to prevent any bending of the 

 great superincumbent mass. The whole stands on a mahogany 

 frame resting on castors. 



The coil is furnished with two primaries^ either of which 

 may be used at pleasure. Either may be replaced by the 

 other by two men in the course of a few minutes. The one 

 to be used for long sparks, and indeed for most experiments, 

 has a core consisting of a bundle of iron wires each *032 inch 

 thick, and forming together a solid cylinder 44 inches in length 

 and 3*5625 inches in diameter. Its weight is 67 lbs. The 

 copper wire used in this primary is 660 yards in length, 

 •096 inch in diameter, has a conductivity of 93 per cent., 

 and offers a total resistance of 2*3 ohms. It contains 1344 

 turns wound singly in 6 layers, has a total length of 42 inches, 



ignition of explosive mixtures of hydrocarbons and air is measured by 

 Bunsen's method. The maximum velocity for marsh-gas and air was 

 524 millim., the minimum 0041 millim. per second. The numbers 

 for coal-gas and air were — maximum 1-01 millim. and minimum 0-097 

 millim. per second. The velocities are in these instances very slow; and 

 the experiments show that they are still further reduced by an excess of 

 either constituent of the mixture. 



