THE CARBOX BUTT OX 



21 



also serve as a means of measuring such small differences of pressure, 

 and thus furnish a comparison between the causes which produced 

 them. The essential principle of the tasimeter is shown in Fig. 6. 



Hll.i! 



J-H 



^ 



E 



Fig. 6. 



A firm standard, A, holds at its upper end a screw which works 

 against a follower, H, to which is attached the metal cup, I. At the 

 base, between two platinum plates, a, a, is the carbon, C ; the plati- 

 num plates are in a battery circuit provided with a galvanometer. 

 Upon the upper platinum rests a metallic cup, D. Between the two 

 cups, I and D, is placed a piece, E, of any material upon which 

 experiment is to be made. The expansions and contractions of E 

 cause changes of pressure upon the carbon, and thus changes of resist- 

 ance in the electric circuit which are indicated by the galvanometer. 

 The screw-head is turned until the initial pressure is sufficient to 

 deflect the needle a few degrees. After the needle comes to rest, the 

 slightest change of pressure will be indicated. The delicacy of the 

 instrument depends largely upon the coefficient of expansion of the 

 material used at E. With a piece of hard rubber, upon which the 

 heat from the hand placed a few inches away is allowed to act, there 

 is a deflection in the needle of a galvanometer which is insensible to 

 the action of a thermopile facing a red-hot iron near at hand. When 

 extreme delicacy is required, a Thomson's reflecting galvanometer is 

 employed in a Wheatstone bridge in the way indicated in Fig. 7. 

 The tasimeter is placed at i, and adjusted to a given resistance. The 

 resistance at a, b, c, is made the same. The galvaaometer is placed at 

 G, and the minutest change of resistance at i is indicated at the gal- 

 vanometer scale. 



The instrument is of service for a variety of uses. It is an excel- 

 lent device for detecting and measuring small and almost inappreciable 

 quantities of heat. In the total eclipse of the sun in 1878, by the aid 



