1909] on Experiments at High Temperatures and Pressures. 557 



held to some extent bj friction against the sides of the cylindrical 

 hole in which it works, and in the consolidation of the graphite — 

 with reduced fluidity, before it actually flows. One has to trust either 

 to the hardness of the ram or to leave a space round it sufficient to 

 allow graphite to escape, when the apparatus follows the lines of 

 Amagat's standard pressure gauge, but the duration of the experi- 

 ment is curtailed by the exhaustion of the graphite supply. A 

 connection has to be applied for the pressure absorbed by the lead or 

 graphite in accordance with the results of the preliminary trial. It 

 is fair to say that no tendency of the ram to stick has ever been 

 noticed- -on the contrary, changes of volume brought about by 

 heating have made themselves evident at once on the pressure- 

 gauge of the hydraulic press. 



When working with any form of carbon there has been no trouble 

 in arranging to heat the body which is being compressed by electrical 

 means. It has been found most convenient to adjust the current to 

 about the value required by means of a resistance —large compared 

 with that of the pressure vessel — ^the latter being short-circuited 

 meanwhile. In making an experiment, the hydraulic press is worked 

 till the desired pressure is attained, and then by opening the switch 

 the current is thrown on to the apparatus. When the magnesia 

 lining begins to melt, the pressure, as shown by the pump gauge, 

 is seen to fall, graphite flows into the magnesia tube, and the pump 

 is worked so as to compensate for this. Under these conditions the 

 pressure is probably transmitted without appreciable loss, as the narrow 

 part of the cylinder is now in a fluid bath. After a sufficient time has 

 been allowed the switch is closed and the pressure kept up by pump- 

 ing till the apparatus is cold. Originally an apparatus with a cyUnder 

 made in one piece was employed, and in this case there was a con- 

 siderable voltage between the graphite entering the apparatus and 

 the steel walls of the pressure vessel. After a few seconds of intense 

 heating it frequently happened that an explosion took place, due (as 

 could be seen by subsequent examination) to filaments of graphite 

 being driven through the magnesia and producing short circuits 

 against the steel vessel. With the construction above described these 

 explosions do not occur, and there is the additional and very real 

 advantage that when an experiment is over the apparatus can be 

 opened in the middle and everything exposed to view. 



A large number of experiments were made on different kinds of 

 carbon and graphite. The weight of material in the highly heated 

 part was generally from 1 to 2 grammes, and the energy supply was 

 at a rate of 5 to 10 kilowatts for from three to six seconds. The 

 pressure in a successful experiment lay at from 50 to 100 tons per 

 square inch throughout. The magnesia lining was usually melted 

 for a distance up to 1 centimetre round the graphite. Now magnesia 

 melts at ordinary pressures at about 2000° C., but the energy supply 



