176 Loi'd Armstrong. On a Multiple Induction 



"On a Multiple Induction Machine for producing High 

 Tension Electricity, and on some remarkable Results 

 obtained with it." By LORD ARMSTRONG, C.B., F.R.S. 

 Received May 18, Read June 16, 1892. 



[PLATES 213.] 



It is now nearly half a century since I undertook to design, for the 

 Polytechnic Institution then existing in London, a large hydro- 

 electric machine, on the plan of the smaller one I had previously 

 designed for myself. It was a very short time in uiy hands after its 

 completion, and I had scarcely any opportunity of afterwards using 

 it in the lecture room of the Institution. My experience with it was, 

 however, sufficient to show me that its great power was very much 

 less wheii used in a room than iu the outside atmosphere when dry. 

 I have ever since entertained the idea of constructing a similar 

 machine of equal or greater power for my private use ; but, until 

 lately, the exigencies of business pursuits precluded my giving atten- 

 tion to the subject. 



On recommencing my experiments on this branch of electrical 

 science, my immediate object was to improve the frictional apparatus 

 of the steam jet, so as to obtain the greatest effect from a given 

 expenditure of steam, while my ultimate intention was to construct 

 a large machine with such a number of jets as would afford me a more 

 copious supply of frictiomil electricity than could be obtained by any 

 other convenient means. I soon found, however, that the difficulty 

 of obtaining effectual insulation in the open air, except under the 

 most favourable conditions of weather, would involve great interrup- 

 tion and disturbing effect, and I therefore turned my thoughts to the 

 induction coil as a source of high tension electricity, that would not 

 only be independent of the caprice of the weather, but would also 

 save me from atmospheric inclemency, which, however harmless it 

 might have been in the youthful days of my hydro-electric expe- 

 riences, could not be safely endured at my present advanced period 

 of life. 



I did not natter myself that 1 could make any important improve- 

 ment upon the Ruhmkorff coil. That remarkable instrument had 

 been so long in use, and had undergone so much development, that 

 its career of progress might well be considered as nearly, if not 

 quite, played out. But although Ruhmkorff coils had been con- 

 structed capable of yielding sparks of unprecedented length, yet it 

 was obvious that the output of electric energy, when estimated in 

 ampere as well as in volt measurement, must be relatively very 

 inferior to that of smaller coils, in which each convolution of the 



