602 PROCEEDINGS OF THE AMERICAN ACADEMY. 



perature but there must be no temperature gradients inside the box, 

 more particularly vertical gradients. So an investigation of the dis- 

 tribution of temperature was made. The mercury thermometer was 

 found to be worthless for this work as it does not show very small 

 changes readily. Accordingly a platinum resistance thermometer 

 made by Mr. C. H. Day was used. This is made of about 50 cm. of 

 platinum wire fused on to platinum leads. The resistance wire is 

 wound on a small mica frame in the form of a cross. The cross was 

 first made and cemented together with "cementium." Two slits 

 were cut in the mica, the platinum wire doubled and the loop in the 

 end caught in the slit. Then the wire was wound on double in small 

 cuts in the mica and finally fused to the platinum leads in the oxyhy- 

 drogen flame. The two thermometer leads together with the com- 

 pensating leads were thrust through small mica discs, and the whole 

 placed in a thin walled glass tube. The tube was slightly enlarged 

 at the top so that it might hang in a hole in a piece of vulcanized fibre. 

 Flexible cords were then soldered to the platinum leads and finally 

 a second piece of fibre was fastened to the first by screws holding tube 

 and leads firmly. The compensating leads are connected in series with 

 a good resistance box and the two sets are connected to a slide wire 

 bridge of the circular drum type made b3' the Leeds and Northrup Co. 



A steady current of .007 amps, is allowed to flow through the ther- 

 mometer so that it is always slightly higher in temperature than its 

 surroundings, but the amount is very small and is constant. The 

 thermometer was calibrated by immersing in melted ice, in steam, 

 and finally in vapor of boiling aniline which had been redistilled 

 several times. The calibration curve compares very favorably with 

 those given by Callendar. As the thermometer is used, one small 

 division on the galvanometer scale corresponds to a change in tempera- 

 ture of about 1/120° so that the thermometer is easil}^ read to 1/1000°. 

 The platinum thermometer surpasses the mercury thermometer in 

 its ability to follow small changes in temperature, and while the scale 

 of this thermometer in the higher region may be in doubt by as 

 much as 1/10°, its efficiency is in no way impaired. During the 

 warming up stages in any experiment, the current for heating is taken 

 from the 110 volt dynamo circuit, but this is too unsteady for accurate 

 work. So when the temperature rises near the desired point the 110 

 volt storage battery circuit is thrown in. For work requiring 1/10° 

 accuracy the lighting circuit is ordinarily steady enough. 



It is extremely doubtful if the readings of most mercury thermom- 

 eters can be relied upon to 1 /100th of a degree when working at 

 temperatures as high as 200°. The amount of stem exposed, sticking 



