[ 435 ] 



LXI. On Thermo-electrical Currents from the Condensation of 

 Vapour, and the Evaporation of Water. By John Mickle*. 



IP we take 12 inches each of iron and copper wire (No. 20 is 

 what I have used) and twist them together at one end, we 

 have a thermo-pair giving very good deflections when the loose 

 ends are attached to a sensitive galvanometer, and the twisted 

 end applied to any source of heat. But by uniting a series of 

 such pairs, in lengths of 6 inches each, a very cheap and useful 

 pile can be constructed capable of appreciating radiant heat from 

 the hand at a distance of 6 inches, and from a luminous source, 

 such as a jet of gas, at a distance of 12 inches. 



I was working with a wire pile of this description, consisting 

 of 40 pairs, endeavouring to obtain minus deflections by blowing 

 upon it through a glass tube 24 inches in length and ^ inch in 

 diameter, but found the deviations produced indicating plus 

 instead of minus — due to heat instead of cold. Knowing that 

 air from the lungs contains much moisture, it appeared to me 

 probable that the deflections proceeded from the heat conse- 

 quent upon the condensation of the vapour upon the wires of 

 the pile. To test the truth of this conjecture, a current of dry 

 air was directed upon the face of the pile from a pair of bellows, 

 but without result. The friction of the air upon the wires of the 

 pile could therefore have nothing to do with the production of 

 the current. Since dry air did not seem to be concerned in the 

 result, my next step was to try vapour. With this object in 

 view, I heated some water in a glass retort and allowed the steam 

 to escape freely from the mouth of the vessel. On bringing one 

 face of the pile into contact with the ascending column of vapour, 

 at a distance of 12 inches from the orifice of the retort, a power- 

 ful deflection of the needle of the galvanometer instantly mani- 

 fested itself. There remained yet another consideration. Would 

 not a voltaic effect be produced by the oxidation of the iron in 

 contact with copper in an atmosphere of steam ? To satisfy this 

 objection, the experiment was repeated with copper and plati- 

 num wires ; but the results differed only in the direction and 

 intensity of the current — the former being contrary, and the 

 latter weaker than with iron and copper wires. 



Assuming that the previous experiments justify the con- 

 clusion that the condensation of vapour originates a positive 

 current, then obviously a negative current ought to result from 

 evaporation. To determine whether this deduction was accord- 

 ant with fact, the following arrangement was adopted. Taking 

 only a single pair of the iron and copper wires twisted together 



* Communicated by the Author. 



