Determining the Mechanical Equivalent of Heat. 165 



ture-coefficient), about J inch wide, O03 inch thick, and about 

 5 feet long, was bent into a series of zigzags (fig. 1) so as to 



Fkr. 1. 



form a kind of circular gridiron, M M, in which the successive 

 portions of strip lie all in one plane, the whole being held 

 rigid by a strip of vulcanized fibre F crossing the gridiron 

 and to which each portion of strip was screwed. Another 

 precisely similar gridiron was placed 3 inches below the first, 

 and they were held together by three thin ebonite pillars E 

 screwed to the strips of vulcanized fibre, the whole forming 

 the top and bottom of a sort of cylindrical box, 5 inches dia- 

 meter and 3 inches high. The two grids were joined in 

 series, and the ends of the strip of manganin were soldered 

 to two copper wires C C, about 0*128 inch thick and 6 inches 

 long, which were insulated from one another by vulcanized 

 fibre separators, and constituted a kind of handle by means of 

 which the open box could be moved up and down in the con- 

 taining vessel. 



The whole surface exposed to the water received a thin coat 

 of varnish to prevent any electrolysis due to the difference of 

 potential between the different parts of the strip. At two 

 points in the same vertical line the zigzags forming the two 

 grids are bent so as to leave a space for the passage of a 

 thermometer t, the bulb of which is midway between the 

 grids when the lower one is resting on the bottom of the 

 vessel. The whole heating surface exposed to the w^ater is 

 about 60 square inches, or 400 square centimetres, The 



