HAYES. — EREORS IN COOLING CURVES. 



13 



Fig. 2,. 



liable to occur between any two curves, as, for instance, between the 

 heating and cooling curves used in determining e. 



If so much variation can be due to change in the contact lag, it 

 would seem that this lag must be great, perhaps furnishing a large 

 part of the entire lag. A consideration of the ordinary form of thermo- 

 j unction should lead one to suspect this, even though the results ob- 

 tained from Curves l' and 2 had not disclosed the fact. 



The two elements forming the couple are usually fused together in 

 the form of a bead nearly spherical in shape. Under the most favor- 

 able conditions this bead rests on the bottom of 

 the protection tube, though ofttimes it takes a 

 position as shown in Figure 2. In either case the 

 contact between the tube and bead is imperfect. 

 In the first case we have a point, or, at most, a 

 very small surface of contact, and in the second 

 case no contact at all. In the first case the j unc- 

 tion can lose its heat by conduction and radiation, 

 though mostly by radiation ; and in the second 

 case its heat must leave wholly by radiation. In 

 either case it would seem that the contact lag must 

 be large, and that it would vary with the position 

 of the junction in the tube. 



ConUict lag is gr('<it. — To test out the lag due to imperfect contact 

 between tube and junction, a protection tube open at the bottom was 

 used. This allowed the bead of the junction to rest directly on the 

 coi)per. Here the lag was due to imperfect contact and lag in the 

 couple itself The lag in the couple was very small, as the material 

 of the couple was a good heat conductor, and because the heat capacity 

 of the couple was small. Several curves were taken with this arrange- 

 ment of the couples, and in each case the lag was nearly as great as 

 that given by the ordinary arrangement with the same rate of cooling. 

 Curve 10, Plate 6, is a typical example. Here the rate of cooling was 

 the same as that employed in Curves 4 and 4a. 



It is to be noticed that the lag is nearly a» great as that registered 

 in Curve 4. This lag is not all due to imperfect contact and lag in 

 the junction, for a thin film of oxide always formed on the surface of 

 the copper, even when the surface was polished before the experiment ; 

 and this oxide is a poor heat conductor. However, the experiment 

 gives conclusive proof that the contact lag is great. 



Means for reducing this lag. — The only way to reduce this lag is to 

 use a ditferent form of junction. The lag can be materially reduced 

 by pounding the bead down to a thin sheet and fitting it to the bottom 



