6 PKOCEEDINGS OF THE AJVIEKICAN ACADEMY. 



were made of copper against constantan, and were so arranged that one 

 gave the temperature just outside the protection tube, while the other 

 gave the temperature from the inside after the usual manner. 



Arrangement of thermo-couples. — The arrangement of these junc- 

 tions was as follows : A cylindrical copper block five inches long and 

 three inches in diameter had two holes, each three inches deep, drilled 

 symmetrically in one end. A constantan wire was fused to the bottom 

 of one hole, thus forming a junction with the copper block as one ele- 

 ment. This couple gave at all times the temperature of that portion 

 of the block which formed the bottom of the hole. Moreover, since the 

 two holes were symmetrically placed and the cooling was made symmet- 

 rical, this junction gave the temperature at the bottom of the other 

 hole. 



In this second hole an ordinary protection tube was placed with 

 a copper-constantan junction inside. The protection tube was sur- 

 rounded with a thin film of lead, so that the tube experienced the same 

 external conditions that are met with in taking the cooling curve of a 

 molten metal or alloy. A protection tube open at the end was placed 

 in the hole first mentioned, so that the heat capacity of the two holes 

 should be the same. 



Because of its short period — less than one fiftieth of a second — it 

 was possible to alternate the galvanometer rapidly between the two 

 thermo-couples. For most work, however, an alternation of about once 

 a second was found satisfactory. 



The cold junction of each couple was kept at the temperature of 

 melting ice, so the deflection of the fiber in the galvanometer was 

 always proportional to the temperature of the hot junction of the 

 couple with which it was in series. This deflection was photographed 

 on a sensitive film which was rotated on the drum of a chronograph. 

 In this way it was possible to photograph on the same film the two 

 curves, one giving the temperature just outside the protection tube, 

 and the other giving the temperature inside the tube, according to the 

 usual manner. These curves were traced by a succession of fine dots, 

 but the alternations were so rapid that in the case of slow cooling the 

 curves appear as an unbroken line. These curves had millivolts for 

 ordinates and time for abscissae, and the apparatus was so arranged 

 that these coordinates were photographed on the film. The difference 

 in height between the two curves at any instant was evidently a meas- 

 ure of the temperature lag due to all three causes, — tube lag, contact 

 lag, and lag in the junction ; but thus far there was nothing to indi- 

 cate what part of this lag was due to each of the three causes. 



The curves of the annexed plates, reproduced from the actual photo- 



