62 Emission and Transmission of Heat 



was obtained in the following manner ; a very thin band of iron 

 four-tenths of an inch wide, and six and a half feet long, was 

 soldered at each end to bands of copper of the same dimensions ; 

 one of the joints and the adjacent portions of the bands were 

 bound to the cylinder of glass, the other joint was secured in like 

 manner to a cylindrical vessel M containing water, an agitator 

 and thermometer and provided with a lamp by which the water 

 could be readily heated, finally the two free ends of the copper 

 bands were connected through a very sensitive galvanometer N. 

 When it was desired to measure the temperature of the surface of 

 the cylinder, the circuit was closed, and the water in the vessel 

 M heated until the needle of the galvanometer returned to zero ; 

 at this instant both joints between the iron and copper bands 

 were at the same temperature and consequently that of the sur- 

 face of the cylinder was the same as that of the water in the ves- 

 sel M. To fasten the joints against the surfaces of the two cyl- 

 inders I used a band of cotton, three quarters of an inch wide, 

 which enveloped the metal band and pressed it against nearly the 

 whole circumference of the cylinder ; the band was held clasped 

 by a buckle, and the free ends of the metal emerged from slits in 

 the cotton. The thermometer serving to indicate the tempera- 

 ture of the water in the vessel whose surface was in contact with 

 the second junction of the bands, was placed horizontally that it 

 might be the more easily read ; its stem being protected by a ver- 

 tical board beneath it ; one could easily estimate one-hundreth of 

 a degree. The vessel was heated by an alcohol lamp. The gal- 

 vanometer made by M. Ruhmkorff was sensitive enough to in- 

 dicate one-fortieth of a Fahrenheit degree difference in the tem- 

 peratures of the two junctions. 



848. The cylinder of glass had no influence on the results, 

 on account of its thinness and the high conductivity of the ves- 

 sel. I also used a paper envelope glued to three tin cyl- 

 inders four-tenths of an inch wide of the same diameter 

 and with the same axis, held in position by two narrow 

 strips of tin (Fig. 172). The strips of tin made it possi- 



ble to fasten closely the metallic band serving to measure Fig. 173 

 the temperature of the surface of the cylinder. This meth- 

 thod gave the same results as the glass cylinder. 



