H.\TES. — ERRORS IN COOLING CURVES. 21 



at least ten such curves could be placed on the same film, and even a 

 greater number when the rate of cooling is slow. 



Copper-constantan couples were used throughout this work, princi- 

 pally for the reason that it made possible the use of a copper block. 

 Several reasons made it desirable to use copper. The copper has no 

 transition points in the temperature range covered, so the cooling 

 curves would have no irregularities due to such transitions. Copper 

 being an excellent heat conductor, the temperature at the bottom of the 

 two holes will be nearly the same, even if there are some slight irregu- 

 larities in the cooling at the surface. Copper and lead is one of the 

 few combinations of metals that do not alloy more or less readily. 

 Lastly, copper against constantan makes a thermo-couple that gives a 

 high thermo-electromotive force. 



Validity of the method employed for determining the lag through the 

 protection tube, etc. — Since the validity of this whole paper depends 

 on the condition that the temperature at the bottom of the two holes 

 shall be the same at all times, numerous tests were made to see if this 

 condition were fulfilled. To test this condition, a constantan wire was 

 fused to the bottom of each hole in the block, and the two curves giv- 

 ing the temperature at the bottom of each of the two holes were taken 

 for various rates of cooling. These curves overlapped in nearly all 

 cases, the variation scarcely ever being more than the width of the line. 

 Curve 14, Plate 6, gives such a set of curves for rapid cooling. The 

 curves show the temperature variation between the two junctions at its 

 greatest. On this curve, one space on the time axis represents three 

 seconds. 



It might be urged that a large portion of the lag recorded in all 

 cases was due to lag through the lead film which surrounded the tube. 

 Curve 13 (Plate 6), a curve resulting from an accident, offsets this argu- 

 ment. Just after the cooling had started, the tube broke and allowed 

 the lead to come in contact with the junction. This break occurred 

 when the temperature of the block was only a little above the melting- 

 point of the lead. After the experiment was over, the bead of the 

 junction was found about half imbedded in the lead. The cooling here 

 was rapid, having been caused by dipping the copper block in ice-water. 

 The irregularities in the curve, and probably the breaking of the tube, 

 were caused by the water boiling over on the top of the block. Despite 

 this, however, the curves coincide throughout most of their length. 

 This proves conclusively that there was little lag through the lead 

 film. 



Furnace and cooling hath arrangement. — Figure 9 shows the 

 arrangement for heating and cooling the copper block. Because of 



