A Detector for Spark in Hertz's Resonator. 299 



This expresses the chance of there being 4 x 10 11 molecules of 

 oxygen in A 5 and 16 x 10 11 in B. Just half this fraction ex- 

 presses the probability that the molecules of nitrogen are 

 distributed in exactly the same proportion between A and B, 

 because the number of molecules of nitrogen is four times 

 greater than of oxygen. 



If n denote the molecules of one gas, and n' that of the 

 molecules of another, the probability that each shall be dis- 

 tributed between A and B in the exact proportion of the 

 volume, is 



1 



2iref *Jnn! 

 The value for the supposed case of oxygen and nitrogen is 



1 1 



27TX-16X4X10 1 * 4021 x 10 9 ' 



which is the result stated at the conclusion of the text above. 



XXX. A Detector for Spark in Hertz's Resonator. 

 By W. Lucas, M.A., and T. A. Garrett, M. A. * 



SINCE Prof. Hertz published his classical researches on 

 electrical oscillations several methods have been devised 

 for showing some of the principal experiments to a number 

 of people simultaneously. 



It seemed to us that a very direct method of doing this 

 would be to make the spark of the resonator ignite an ex- 

 plosive mixture of gases. After making many experiments 

 we find that a very convenient form of apparatus for effecting 

 this is the following : — A long strip of wood (not shown in 

 the figure), having near its ends two projecting blocks with V- 

 cuts in them, is supported in a horizontal position, and to the 

 middle of it is screwed the base-board A, its position being 

 vertical. B and B' are two more blocks with V-cuts in them. 

 The four blocks are arranged in such a way that the V-cuts 

 in them are in the same straight line. In the V-cuts of the 

 four blocks the two straight wires which form the resonator 

 rest. The spark-gap Gr is between the ends of the platinum 

 wires P and P' ; these are led down into the V-cuts in B and 

 B' as shown, so that the resonator wires rest in contact with 

 them. P is fixed firmly to the wooden lever L, so that the 

 size of the spark-gap can be easily adjusted by turning the 

 milled head H, against which the lever is pressed by the 



* Communicated by the Authors. 



