164 Prof. G. Wiedemann on the Laws of the 



While the foregoing researches lead us to some conclusions 

 respecting the quantity of electricity necessary for the begin- 

 ning of a discharge, it is requisite through a measurement of 

 the heat developed, to learn something definite about the pro- 

 gress of the discharge during the passage of the electricity. By 

 application of a thermopile to the different sections of the ca- 

 pillary tube, the rise of temperature due to the quantity of 

 heat developed in a unit of time may be determined. These 

 quantities of heat in various sections of a capillary tube are 

 the same whether the tube be long or short, even when the 

 length is varied in the proportion 1 : 1*78 ; only in longer 

 tubes the heating, and with it the brilliancy of the discharge, 

 diminishes towards the electrode connected with earth, especi- 

 ally when it is negative. 



VI. Tubes of various internal, but equal external diameter 

 receive, under circumstances otherwise similar, nearly equal 

 quantities of heat in equal times from the discharge, even when 

 the cross section of the space through which the discharge 

 passes is increased in the proportion 1 : 4. This condition of 

 things is maintained when capillary tubes of different diame- 

 ters, or when tubes of diameters varying at different parts, are 

 used. 



If the capillary tube is slightly heated at any part, the 

 number of discharges scarcely alters. If it be considerably 

 heated, a bright sodium light appears at the heating-point, 

 and the number of discharges increases. But the sodium 

 light does not extend beyond the heated part. If the space 

 about the electrodes be heated, a sodium light is similarly de- 

 veloped without spreading further ; the intervals of the dis- 

 charges increase. But as soon as the pressure due to the 

 heating reverts to what it was before, the intervals resume 

 their former value, even though the glass be heated almost to 

 fusion. 



The charge on a capillary tube, examined electroscopically 

 throughout its entire length, is similar to that of the electrode 

 connected with the machine, provided that the other electrode 

 be put to earth. It diminishes gradually towards the latter 

 electrode. If both electrodes are connected with the machine, 

 the charges on the capillary tube diminish from each end to a 

 neutral point in the middle. 



With a view of obtaining some information about the 

 nature of the dark space near the negative electrode, some 

 experiments were made upon the effect on the discharge 

 due to an interruption of the circuit by a slight interval of 

 air. For this purpose a cylindrical tube was used. Its length 

 was 30 centims., and breadth 30 millims. ; it contained rarefied 



