244 Professors W. E. Ayrton and J. Perry on 



The cylindrical hole in the stone through which the wires were 

 inserted was only made just large enough to receive them ; and 

 the possibility of water entering the hole and making contact 

 with the junction was prevented by the surface of the insu- 

 lated wires being smeared over with a paste composed of white 

 lead, red lead, and linseed-oil, which by hardening cemented 

 the wires to the stone. The copper wire passed to a key, K, 

 which was connected with one terminal, T, of a delicate dead- 

 beat reflecting-galvanometer of about three quarters of an ohm 

 resistance, which we had constructed for measuring thermal 

 currents. The other end of the iron wire was bound and sol- 

 dered at J to another copper wire, which was connected with 

 the other terminal, T', of the galvanometer. D D was a copper 

 compensating water-bath, by means of which the thermo- 

 electric junction J could be always kept at the same, or nearly 

 at the same, temperature as the other junction at the centre of 

 the ball. To ensure the junction J quickly acquiring the tem- 

 perature of the water in the bath D D, a small perforated plate 

 of copper, P, was soldered to J and hung in the water with- 

 out touching the sides or bottom of the bath. The bath was 

 divided longitudinally by a perforated copper plate, Q Q, to 

 allow of the water being kept rapidly stirred (to ensure equa- 

 lity of temperature) without risk of the Kew standard thermo- 

 meter S, which hung in the water, being broken. 



This bath was fitted with two taps, U, V — the one for letting 

 in cold water from the cistern, the other for emptying the bath. 

 H is another thermometer hanging in the bath B, and having 

 its bulb surrounded by a small metallic screen to shield it 

 from direct radiation from the ball, but which allowed of free 

 access of the water to the bulb. Either water-bath could be 

 heated by suitable spirit-lamps. 



III. Method of experimenting. — The two water-baths having 

 been filled with water and left for a sufficiently long time for 

 the temperature of all parts of the apparatus to have become 

 uniform, or very nearly so, a reading of the galvanometer was 

 taken, a small deflection d ± being obtained. This deflection 

 was due to a small unknown difference of temperature t± still 

 remaining between the two junctions. The temperature of J 

 was now raised by a number of small increments, t 2 , t 3 , &c, 

 producing deflections d 2 , d 3 , &c. respectively ; then, since for 

 small differences of temperature the currents are proportional 

 to the differences, we have, if T is the difference of temperature 

 corresponding with any small deflection D, 



^2 , ^3 — ^ 2 , ^4 + ^3 + ^2 



m q d 2 — d 1 d 3 — d 1 d 4 —d l 



