212 Mr. J. Prideaux on the Theory of Voltaic Action. 



To put Dr. Hare's charcoal experiment to a test as nearly 

 parallel as the cases seem to admit, — 



A card was placed between two copper plates, bound round 

 with wire, and set in the fire until the card was well charred. 

 It was then taken out and put between two plates of polished 

 copper, to each of which a wire was soldered ; they were then 

 bound round tightly with many turns of waxed thread, and a 

 wooden wedge afterwards thrust under the thread, on each 

 side, so as to insure the charcoal being firmly and uniformly 

 pressed between the plates. The two wires were then made 

 the connexion between a thermotest and a magnetest, one 

 connected with each pole of a large voltaic pair, weakly 

 charged. Neither instrument was distinctly affected. 



A multiplier being substituted for the magnetest, the needle 

 deviated a few degrees. A slip of bright copper was now 

 doubled so as to pinch the plates containing the charcoal, and 

 thus complete the metallic communication. The needle was 

 set spinning, and the thermotest rose 10° an inch. 



The charge was now increased to about ^V tn °f nitric acid, 

 a smaller pair being employed (30) ; the charcoal and slip of 

 copper forming the communication alternately ; and the mul- 

 tiplier being again superseded by the magnetest. The results 

 are given in the following table. 



(The column headed " Mag." is the deflection of the needle. 

 — " Curr." is the intensity of the current, calculated from the 

 deflection, BecquerePs table. — Ann. de Chim. et de Phys. 

 for August 1829. — " Ther." is the rise of the thermotest.) 



Through Charcoal. 

 Mag. Curr. Ther. 

 25° 16 1-5 



30 23 2 



25 16 1 



25 16 2 



Mean 27 18 1*6 



Through Copper. 



Mag. Curr. Ther. 



48° 60 8 



50 68 8 



48 60 8 



46 54 7 



48 61 8 



Here the electric current is obstructed by the charcoal, as 

 well as the calorific; and the calorific effect is, in each case, 

 in such proportion to the current as would be expected, con- 

 sidering that weak currents which pass through the wire with- 

 out resistance do not heat it at all ; whence the heat must 

 increase at a greater rate than the current. 



Thus it is seen that charcoal obstructs the electric as well 

 as the calorific current; and that common electricity is more 

 effectually deprived of its henting power by passing through 



