308 M. F. Stenger on the Electric 



when the arc has been interrupted immediately after its 

 establishment. Most of these experiments have been made with 

 an extremely simple form of apparatus, which permits working 

 with pure gases in very various pressures, since the movement 

 of the electrodes can be effected without the use of stoppers, 

 which are never absolutely air-tight. The apparatus (fig. 1 ) 

 consists of a bulb with two tubes blowm on to it ; each of the 

 tubes is further provided with a tube at the side, of which the 

 one communicated with aBessel-Hagen air-pump and the other 

 with the apparatus for evolving the gas. The ends of the 

 tubes are provided with caps fitted by grinding, carrying the 

 electrodes, and the electrodes may be brought into contact 

 and separated again, so to form the arc, by simply turning 

 these caps. 



This arrangement, however, could not be used for experi- 

 ments with an arc of greater length. In these cases I have 

 employed another arrangement, represented in fig. 2 in simple 

 form. A tube (b) 1 centim. wide and 80 centim. long, was 

 melted on to the centre bulb-shaped part a, and communicated 

 by means of an india-rubber tube w r ith a second tube V of 

 similar dimensions. The short side-tube c communicated 

 with the mercury-pump. At the top a short tube d, also 

 1 centim. wide but 10 centim. long, was melted on, upon which 

 a wide tube e was placed, from which lastly a tube/, 80 centims. 

 long, led downwards and was connected with another (g) by 

 means of an india-rubber tube. One of the two electrodes (the 

 apparatus was used only with carbon points) was inserted in 

 the tube d, so that its end reached to the middle of the bulb- 

 shaped portion, and was held in position by means of four 

 copper bars, attached to a copper ring which was clamped 

 close to the carbon. The wide tube e was then melted 

 together at the top. The current was conducted to the 

 upper carbon by means of mercury which filled the system 

 of tubes /, g and the space between c and d, and surrounded 

 the four copper wires. The lower carbon floated on mercury 

 in the tube 6, so that the distance between the carbons could 

 be altered by simply raising or lowering V. 



It appeared to me of importance by means of this apparatus 

 to control the statement of Grove, that in a vacuum the two car- 

 bons show no difference or very little difference in temperature. 

 For this purpose air, which had been repeatedly and care- 

 fully dried, was introduced into the apparatus and exhausted by 

 means of the air-pump to a pressure of less than ^ millim. 

 As soon as the current of the Gramme machine had established 

 the arc-light, a considerable increase of pressure took place 

 in consequence of evolution of gas from the glowing carbons. 



