HAYES. — ERRORS m COOLING CURVES. 17 



Placing the time coordinates. — The chronograph is so arranged 

 that it can be run with either of two motors 1 or 2 (Figure 4). One- 

 tenth horse power A. C. motors are used for this work, as they give to 

 the drum a more uniform rotation than the ordinary clockwork arrange- 

 ment. Motor 2 is connected to a variable gear, such that the time of 

 rotation for the drum can be varied from forty seconds to two hours. 

 This motor is used to turn the drum whenever a cooling curve is being 

 photographed, the gearing always being so adjusted that the drum 

 rotates about once while the cooling takes place. Box T contains an 

 arrangement for throwing a periodic flash through Si of the galva- 

 nometer. This light finally passes through the slit in the screen and 

 gives a fine white line on the film. If the drum rotates uniformly and 

 the flashes occur at the end of equal periods of time, and if at the 

 same time a curve is traced by the spot of light coming from the fila- 

 ment, we should have, upon developing the film, a temperature vs. 

 time curve with the time coordinates drawn. 



Flaring the e. m. f. coordinates. — By means of the double switch R 

 a potentiometer P can be thrown into circuit with the galvanometer, 

 and this instrument can then be made to register millivolts. This 

 arrangement makes it possible to place electromotive force coordinates, 

 on the film. First rotate the drum with no e. m. f. through the gal- 

 vanometer. The spot of light then traces the zero line on the film. 

 Then repeat this operation for each millivolt, or whatever increment 

 of e. m. f. is desired, until the range of e. m. f. covered by the thermo- 

 couple is passed. For this work motor 2 is used, as the gearing here is 

 such that the drum rotates about once in ten seconds. 



In order that these coordinates should have the same intensity as 

 the curve, the time of exposure should be the same. This causes a 

 great loss of time, as the period of rotation for the drum is more than 

 one hour when the cooling is slow. This waste of time is avoided by 

 widening the slit S while these coordinates are being made. By making 

 the slit one millimeter wide, the time required for recording one coordi- 

 nate line is about ten seconds. Thus the total time required for add- 

 ing the e. m. f coordinates to one of the curves of this paper is about 

 ten minutes. This method for adding the e. m. f. coordinates amounts 

 to calibrating the galvanometer for each curve, so that the results are 

 largely insured against variation in the galvanometer. 



This recording apparatus is essentially the same as that devised by 

 Einthoven and described at length in Annalen der Physik, Ut(>3, Vierte 

 Folge, Band 12. It is without doubt the best form thus far devised for 

 recording rapid cooling, but the expense of the apparatus has tended to 

 prohibit its general use. It therefore seems desirable to describe briefly 



VOL. XLVII. — 2 



