184 Prof. G. Wiedemann on the Laws of the 



mencement of the discharge ; the influence of the electricities 

 of the electrodes on one another will therefore, under similar 

 circumstances, decrease ; and the discharge will consequently 

 depend more exclusively upon the surroundings of the elec- 

 trodes. Nearly equal quantities of electricity are discharged 

 in tubes of different lengths. 



c. Resistance at the negative electrode. — If we were inclined 

 to infer that the supposed resistance of the positive discharge 

 compared with the negative is, under certain circumstances, 

 small, Ave might determine the supposed resistance of the 

 halo by measuring the resistance of the tube with different 

 negative electrodes. 



1. Influence of the size of the electrodes. — If two similar 

 tubes with positive electrodes of the same length, and nega- 

 tive of the same thickness but different lengths, are inserted 

 side by side in the circuit of the coil, then, according to 

 Hittorf, when the rarefaction is such as to produce a halo 

 covering completely both the negatives, the intensity in the 

 two branches, setting aside the influence of the positive elec- 

 trode &c, will be nearly in proportion to the length of the ne- 

 gative electrode. In this case, if the induction-current gives a 

 quantity of electricity sufficient to maintain the whole surfaces 

 of the two electrodes at a tension necessary for a discharge, and 

 therefore under similar conditions when the discharge takes 

 place, then the quantity of electricity discharged from all their 

 parts must (last paragraph) be in proportion to their surface. 

 The final result is therefore the same as it would be on the 

 supposition that the resistance of a tube was approximately in- 

 versely proportional to the surface of its negative electrode. 

 In this, however, we have set aside the influence mentioned 

 by Hittorf, of the positive electrode and of the greater cooling 

 of the gas on a very small negative electrode ; in this case, 

 according to our explanations, the gas driven away will be 

 more quickly replaced, and therefore the passage of electricity 

 ceases sooner after the beginning of the discharge, and the 

 resistance appears proportionally too great. 



That the same thing does not occur when the tubes are in- 

 serted separately in the circuit and the intensity of the current 

 is measured, must arise from the fact that with an equal flow 

 of electricity the shorter electrode sooner obtains the tension 

 necessary for the commencement of a discharge than the 

 longer, consequently the discharge also begins sooner ; there- 

 fore the quantities of electricity which flow through one or 

 other of the tubes are not so very different from one another. 

 If the intensity of the current is brought, by the insertion of 

 tubes of sulphate of zinc instead of vacuum-tubes, to the same 



