14 PROCEEDINGS OF THE AMEEICAN ACADEMY. 



of the tube ; but even then the contact lag forms a large part of the total 

 lag. A much better form of couple is one in which the tube forms one 

 element, with the other element fused to the inside bottom of this tube. 



Such a couple has been patented by C. B. Thwing, and is made by 

 James G. Biddle, Philadelphia. This form of junction, as marketed, 

 has the outer element and tube made thick and heavy, in order to pro- 

 long the life of the junction which comes in direct contact with the 

 molten metal. The junction is useless for accurate work on alloys, as 

 it contaminates the melt ; but the form of the junction is correct since 

 the tube lag is made small, the tube being a good heat conductor, and 

 the contact lag is entirely eliminated. 



Improved form of thermo-couple. — The author makes use of a couple 

 of this form, but places a thin coating of lime on the outside of the 

 tube to avoid contamination. The metal tube which forms one element 

 of the junction is made very thin, thus reducing the lag in the couple 

 itself The tubes are deposited electrolytically on a wax form ; the 

 wire leading from the tube, and the second element leading from the 

 bottom of the tube, having been placed in position on the wax form. 

 These wires are thus sealed to the tube by the process of deposition. 

 In this manner the couples can be made nearly any desired size. 



The temperature lag in these junctions is reduced to a minimum. 

 The lime coating can be made very thin, thus causing little lag through 

 the protection tube. The contact lag is small and constant, since the 

 whole surface of the couple is in close contact with the lime coating. 

 Finally the lag in the junction itself is nearly zero, as the heat capacity 

 of the j unction is nearly zero. 



Comparison of old and improved form. — Curve 11, Plate 5, really 

 two separate curves on the same sheet, gives a comparison of the lag 

 due to the two junctions. The curve on the left was taken under the 

 most favorable conditions applicable to the ordinary method. The 

 protection tube was made of thin quartz, and the j unction rested on 

 the bottom of the tube. The curve on the right shows the correspond- 

 ing lag when the tube-form of couple is used. Here the apparent lag 

 is more than the actual lag, as the tube couple gave a higher e. m. f 

 than the couple of which the copper block formed one element. The 

 rate of cooling was the same for both curves, and was obtained by 

 immersing the copper block in oil. 



In all the curves taken, the protection tube was surrounded with a 

 thin film of lead ; yet the cooling curves, except faintly in Curve 1', 

 give no indication of the freezing point of the lead. This point, how- 

 ever, is plainly shown in the cooling curve taken with the tube-form of 

 couple, even though the rate of cooling was high. 



