under the Influence of the Electric Discharge. 359 



the calorimeters, and arrangements for exhausting ; (3) the 

 arrangements for measuring the number of discharges. 



1. The source of electricity employed was usually a Topless * 

 electrical machine with 20 revolving disks, which was driven by 

 a small Schmidt's water motor. In order to have a constant 

 stream of water, most of the experiments were performed at 

 night. In this way a constant velocity of revolution, and a con- 

 stant deflection of the galvanometer included in the circuit, 

 could be maintained for hours together. 



The machine has a few drawbacks, in consequence of the 

 powerful evolution of electricity. First, it attracts the dust 

 very largely, the disks become dirty, and the production of 

 electricity diminishes or even ceases altogether ; moreover 

 large quantities of ozone are produced. Both of these defects 

 are remedied pretty w T ell by covering the machine over with 

 a wooden cover with glass sides, and placing vessels of linseed- 

 oil under it f After use for several weeks the disks become 

 dirty in spite of all precautions, and must then be washed, 

 either with water or with alcohol, by means of a sponge. 

 Simple brushing is of no use. 



In damp weather, and when the disks have become dirty, 

 the direction of the current frequently becomes reversed. This 

 may be prevented tolerably completely by warming the ma- 

 chine. A further peculiarity of this machine is that almost 

 always when it has been allowed to run for a time, then 

 stopped, and again put into action, the direction of the current 

 is the opposite to that obtained at first. This was often very 

 convenient in my experiments, since the electrode connected 

 with the machine was generally wanted to serve first as a 

 positive, and then as a negative electrode. 



In order to introduce sparks of known length into the cir- 

 cuit, a spark-micrometer with spheres of 33 millims. diameter 

 was employed. If no spark was to be introduced, the hori- 

 zontal adjustable brass bars which carry the spheres were con- 

 nected by a brass stirrup. 



The reversal of connexions was always made on the machine 

 itself, copper wires covered with gutta-percha being used for 

 the connexions. One pole of the machine was always con- 

 nected direct to earth, the other connected to the one insulated 

 electrode of the discharge-tube, the other electrode of which 

 was connected with the earth through the galvanometer. 



* Topler, Berl. Ber. 1879, p. 950 et seqq. ; Beibl iv. p. 398, 1880. 



f The use of turpentine is not to be recommended, as it gradually forms 

 a sticky substance on the disks, which destroys the varnish. This sticky 

 substance probably consists of oxidation-products of turpentine, as the 

 disks themselves may be washed with turpentine without damage. 



