HAYES. — ERRORS IN COOLING CURVES. O 



of the crucible might diffuse through the thin porous linings : but 

 with linings two millimeters or more in thickness there can be little 

 or no danger from this source. 



The perfection of this crucible now makes it possible to prepare an 

 alloy of definite constitution, and this removes one of the main diffi- 

 culties in the study of alloys. 



Investigation of the Errors in Cooling Curves. 



More information concerning an alloy can be had from its cooling 

 curve than from any other one source, providing the cooling curve is 

 accurate ; and more information can be had from the temperature-time 

 curve than from any other of the various forms. The difficulty of get- 

 ting such curves is not great providing the cooling is slow ; but when 

 one attempts to take a curve for rapid cooling or chilling numerous 

 difficulties arise. The remainder of this paper is devoted to a con- 

 sideration of these difficulties, the errors that as a result have crept 

 into some of the work on metals, and methods for overcoming these 

 difficulties. 



The errors. — The difficulties that arise when one attempts to take 

 a cooling curve where the cooling is rapid, and the errors that arise 

 therefrom, are due to temperature lag and lag in the galvanometer. 

 Temperature lag takes place through the protection tube, through 

 imperfect contact between the tube and the enclosed thermo-couple, 

 and finally through the couple itself. Lag in the galvanometer is a 

 function of the period of this instrument, probably nearly proportional 

 to the period. This lag, which for simplicity will be called the elec- 

 trical lag, can be almost entirely eliminated by using an Einthoven 

 form of galvanometer (a fine conducting filament suspended in a strong 

 magnetic field), for here the inertia of the moving parts is reduced to 

 a minimum, and the period can be made very short without great loss 

 of sensitiveness. By thus eliminating the electrical lag, I have been 

 able to study the temperature lag in its various phases. The work has 

 been carried out in much the following order : 



a. Lag due to all three causes, — tube lag, contact lag, and lag in 

 the junction. 



b. Contact lag and lag in the junction. 



c. Lag in the junction itself 



Method. — The apparatus used in this work, which is described in 

 detail farther on, consisted essentially of an Einthoven galvanometer 

 with a commutator arrangement, such that this instrument could be 

 thrown in series with either of two thermo-couples. These couples 



