Mr. T. C. Mendenhall on Edison's Taximeter. 115 



face-velocity of about 4*5 feet per second. The experiment 

 (fully detailed in the Roorkee work*) was a very delicate one, 

 on account of the incessant slight oscillations of the water- 

 surface ; but every possible care was taken. The result of 

 twelve trials on one day and twenty-four on another, was that 

 the mean water-surface (i. e. average of the oscillations) at the 

 centre and edges was (on a calm day) most probably level. 



XII. On the Influence of Time on the Change in the Resist- 

 ance of the Carbon Disk o/' Edison's Tasimeter. By T. C. 

 Mendenhall, Columbus, 0.\ 



ABOUT five years ago Edison announced the discovery of 

 the remarkable property possessed by carbon when pre- 

 pared 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 pro- 

 mising 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 Expedi- 

 tion 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 American 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 adjust- 

 ment of the tasimeter so that the galvanometer-needle would 

 remain at zero, and to secure its return to that point after it 

 had been deflected. In fact, the zero adjustment was only 

 made by the use of a peculiar shunt of variable resistance 

 ingeniously contrived by Mr. Edison for the purpose. 



The writer is not aware of any other systematic attempt to 

 secure quantitative results through its use; and, as far as 

 known, the instrument has been generally regarded as pecu- 

 liarly inconstant and unreliable in its indications. 



Having in his possession a tasimeter constructed after the 



* Op. cit. vol. i. chap. viii. arts. 4, 4 a. 



t From Sillintan's American Journal of Science for July 1882, having 

 been read, by invitation, at the April meeting of the National Academy. 

 12 



