T. ('. MmdenhaU—Mu 



[Read, by invitation, at the April meeting of the National Academy.] 



About five years ago Edison announced the discovery of the 

 remarkable property possessed by carbon when prepared in a 

 special manner, in virtue of which its electrical resistance was 

 greatly lessened by subjecting it to an increase of pressure. 

 Among the numerous interesting applications of this discovery 

 which were quickly made, none was more promising or more 

 interesting than the Tasimeter devised by Edison himself. The 

 extreme sensitiveness of the carbon to the slightest changes in 

 pressure gave rise to the hope that the instrument would far 

 exceed in delicacy those previously in use for the detection of 

 minute quantities of heat. 



Mr. Edison was a member of the Draper Eclipse Expedition 

 in the summer of 1878, and used his Tasimeter during the total 

 eclipse of July 29 in that year, attempting to measure the heat 

 emitted by the sun's corona. His report to the director. Dr. 

 Henry Draper, was published in the Proceedings of the Ameri- 

 can Association for the Advancement of Science for the same 

 year. This report shows that the attempt was by no means as 

 successful as could have been desired, the principal obstacle 

 being apparently the difficulty in the adjustment of the Tasi- 

 meter so that the galvanometer needle would remain at zero, 

 that point after it had been deflected. 



secure quantitative results through if- w->\ ai d. , s far as known, 

 the instrument has been generally regarded as peculiarly incon- 

 stant and unreliable in its indications. 



Having in his possession a Tasimeter constructed after the 

 model of that described in the report referred to above, the 

 writer undertook a short time ago to investigate the quantita- 

 tive relation between pressure and resistance for the earbon 

 disk winch belonged to it. In a series of preliminary experi- 

 ments, the use of the toothed wheel and screw, by means of 

 which the pressure is communicated to the disk-, was found to 

 be extremely objectionable on account of the impossi 

 exaeth reproducing a given pressure. This portion of the 

 removed, and an arrangement 

 made by means of which any definite pressure might be quickly 



