108 On a Condenser of Variable Capacity. 



over a number of needles carelessly thrown on water, and 

 witness them approaching and, one after the other, entering 

 into the structure of that geometric figure which conforms to 

 the number of magnets composing it. 



XIY. A Condenser of Variable Capacity, and a Total-Reflexion 

 Experiment. By C. V. Boys, A.R.S.M., Lecturer for the 

 Term on Natural Science at Ujypingham School*, 



WISHING to show my pupils the effect of condensation 

 on the spark, I thought a condenser the capacity of 

 which could be reduced gradually to nothing would be most 

 suitable. So I made this simple contrivance, which answered 

 its purpose well : — 



A glass tube is sealed at one end and is covered with tinfoil 

 for one third of its length ; this forms the outer coating. The 

 inner coating consists of a test-tube with the rim cut off, also 

 covered with tinfoil ; this is fixed to a wire, and can be drawn 

 in and out. When it is fully in, the condenser has its maxi- 

 mum capacity ; when drawn out as far as possible, the two 

 coatings are too far apart to have any sensible action, and the 

 capacity is zero. 



On hanging this on the conductor of a Holtz machine the 

 effect on the spark is well shown. Let the wire be first pushed 

 in as far as possible, the condenser then acts to its full extent ; 

 but on gradually drawing it out the sparks are less and less 

 bright, but follow one another more and more rapidly, till at 

 last, when it is fully out, they have passed gradually to the 

 almost continuous pale spark so characteristic of a Holtz ma- 

 chine. To show the effect best, the poles should not be more 

 than about half an inch apart. Of course much ozone is 

 formed inside the tube. 



The total-reflexion experiment was an accident. A small 

 condenser made of a test-tube gave way under the strain, a 

 minute hole being pierced in the bottom, through which sparks 

 passed almost continually. No light could be seen anywhere 

 except on the rim of the tube, which formed a brilliant circle 

 of light. The light from the spark was totally internally 

 reflected in the substance of the tube till it reached the rim, 

 which it struck normally. The bright circle of light (the tube 

 itself being dark) was very striking ; and the experiment is a 

 far truer illustration of total internal reflexion than the more 

 beautiful one with a stream of water. The tube is, unfortu- 

 nately, broken ; and I have not succeeded in piercing another 

 with the spark. A crack made with a hot wire does not do so well. 

 * Communicated by the Physical Society. 



