44 T. C. Mendenhall^Edisorts Tasimeter. 



brought to bear upon the disk or removed from it. A slender 

 brass rod was placed in a vertical position upon the center of the 

 upper contact piece, the upper end of which rested lightly in a 

 small conical cavity made on the under side of the scale-pan of 

 a balance. The weight was suspended above by a fine thread 

 passing over a pulley, so that by raising or lowering it the 

 pressure was applied or removed as was desired. The carbon 

 disk was made one of the branches of a Wheatstone's Bridge 

 as described by Mr. Edison. In lowering the weight care was 

 taken to make the movement slow enough to avoid an 

 to the disk. When the apparatus stood with the weight lifted, 

 the adjustment of the galvanometer to the zero was made with- 

 out any difficulty, the resistance of the disk appearing to be 

 quite constant. When the pressure was applied, however, the 

 adjustment became very troublesome, and after a few trials it 

 was discovered that time was a very important element in the 

 problem. The addition of a pressure of fifty grams reduced 

 the resistance to nearly one fourth of what it was in its normal 

 condition instantly, but it was found that the minimum was not 

 reached at once. The resistance continued to fall during the 

 first two or three minutes with considerable rapidity and after 

 that more slowly. A series of experiments was accordingly 

 undertaken for the investigation of this phenomenon. After a 

 number of trials, the bridge was adjusted so that when the key 

 was closed simultaneously with the application of the pressure 

 the needle of the galvanometer would remain momentarily at 

 zero, for the instantaneous effect of this pressure seemed ' 



In a few seconds, however, the dpi 

 _ that the resistance was diminishing. With this 

 it was. of course, difficult to ob- 



, showing that the 



tain balances which were . generally oi 



be obtained within, B i ication of the pressure, 



and another a minute or two later, and so on. The operation 

 was repeated many times, and a number of points for the curve 

 shown below were obtained, which, though necessarily some- 

 what scattering, were so situated as to render its general form 

 almost certain. In almost every instance immediately after the 

 removal of the pressure, the normal resistance was again meas- 

 ured, and it was found that while time was necessary for the 

 resistance to reach a minimum after the application of the 

 pressure, the disk seemed to recover its maximum normal 

 ! \-i.starn e inst mtb ipon it- removal. 



After the construction of the curve showing the relation 



between time and resistance, and on the supposition that it 



correctly represents that relation, it was easy to know what the 



it of the bridge should be at the end of any given 



time, and thus the difficulty of that adjustment disappeared. 



