20 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



junction is placed, and a constantan wire is passed through the open 

 tube and fused to the copper block. A copper wire w is fused to the 

 top of the copper block, and protected from the hot gases of the furnace 

 by means of a short porcelain tube. The space between the closed 

 tube and the copper block is filled with lead. Thus the closed tube 



Fig. 9. 



experiences the same conditions that are met with in taking an ordinary 

 cooling curve for a molten metal or alloy. 



The commutator. — The commutator arrangement, by means of 

 which the two thermo-j unctions are alternately thrown in series with 

 the galvanometer, is shown in c. Figure 3. It might be added that 

 several curves could be photographed on the film by simply changing 

 the form of the commutator. The alternations were about once a 

 second throughout this work, but the period of the galvanometer was 

 only one fiftieth of a second, and the damping very rapid, so that as 

 many as five alternations per second could be made. This means that 



