1880.] an Induction Coil ivith a De Meritens Machine. 177 



the striking distance was near the limit that it could attain when no 

 magnetic field was present. If a plate of glass was interposed 

 between the poles of the magnet (which were still used as terminals) 

 the yellow flame disappeared, and the spark divided itself into 

 numerous ramifications of true sparks which found their way round 

 the edges of the plate. As soon as the magnet was excited the 

 resistance in the field became so great as to exceed that of the glass 

 plate itself, and the plate was pierced. 



Professor Dewar was good enough to measure the efficiency of the 

 secondary discharge, by taking an inch spark in a glass bulb placed in 

 the centre of a calorimeter, in the same way as he had already 

 measured the efficiency of the intermittent current direct from the 

 machine. The former amounted to about 430 gramme-units per 

 minute, while • the latter had been found to be 6,000 per minute. 

 The relative efficiency may, therefore, be taken at about 1 : 15. And 

 as the machine was giving about 300 currents per second, this would 

 give for the secondary 



430 : 60 x 300 = -023 unit per discharge, 



and for the primary 



6000 : 60 x300=*3 unit per discharge. 



The results in the case of the secondary discharge may not perhaps 

 possess any great degree of accuracy ; but, in the absence of any other 

 information on the question, they may serve to indicate the general 

 scale 6f magnitude by which the coil discharge is related to that of 

 the machine. 



Leaving the subject of the spark from the induction coil, one of the 

 most remarkable effects produced by this machine was the illumina- 

 tion of vacuum tubes by the currents taken simply from the machine. 

 A small sphere of about two inches in diameter, with an air- vacuum, 

 and having two parallel straight terminals reaching nearly across the 

 sphere and about half an inch apart, was (after the first attempt, 

 when there was some difficulty in getting the discharge to pass) 

 readily illuminated. Owing to the alternate currents, both terminals 

 were of course surrounded with the usual blue halo. When the speed 

 of the machine was reduced, the discharge through the tube was not 

 maintained, showing that only that part of the current from the 

 machine which possessed the highest electromotive force, and perhaps 

 also the greatest strength, was sufficient and was therefore actually 

 used for the purpose. As this was apparently only a small fraction 

 of the whole current, we may herein find an explanation of the fact 

 that, compared with the effect from the induction spark, the illumina- 

 tion was moderate, and the heating insignificant. It would perhaps 

 not be easy to establish an accurate comparison between this and other 

 sources of electricity ; but some idea may be conveyed by the fact 



