40 



THERMOMETER AND PYROMETER. 



form of a letter U, when the balls are 

 unequally heated. This difference is 

 measured by the motion of a coloured 

 liquid contained in the bent tube. " In 

 ordinary cases," says Leslie, " the in- 

 termediate liquor would continue sta- 

 tionary ; for the air in both balls having 

 the same temperature, and, consequently, 

 the same elasticity, the opposite pres- 

 sures would exactly counteract each 

 other ;" but if one ball becomes colder 

 than the other, " it is manifest that the 

 liquor would be pushed towards it by 

 the superior elasticity of the air included 

 in the other." This is the principle 

 which suggested to him the 



1. Differential Thermometer, used 

 with so much skill and ingenuity in those 

 delicate investigations on heat, with 

 which he was occupied from the above 

 period, until the publication of his work 

 early in 1804. The differential thermo- 

 meter in its most usual form is repre- 

 sented in (fig, 36,) where a, b, are two 



Fig 36. 



equal glass spherules, connected by the 

 tube c, d, e,f, slightly dilated just below 

 the ball a, and at e, and partially filled 

 with a coloured liquid, as represented in 

 the figure. The dilatation below a, is 

 intended as a reservoir of liquid ; and 

 that at e, for the more easy adjustment 

 of the liquor to the commencement of 

 the scale, by passing bubbles of air from 

 one ball to the other. The liquid re- 

 commended by Leslie,* after many 

 trials, is strong sulphuric acid, tinged 



k * Experimental Inquiry, p. 4J7, 1804. 



by carmine. The scale he adopts, is 

 millesimal, from the freezing to the 

 boiling point of water ; or 10 degrees of 

 it are equal to one of the scale of Cel- 

 sius. The instrument is cemented to a 

 wooden foot, either immediately, or is 

 furnished with a sliding stem to adapt 

 it to different heights. Each leg of the 

 instrument is usually from 3 to 6 inches 

 in length, and the balls are from 2 to 4 

 inches apart. The calibre of the stem 

 e,f, is from the 1-5 Oth to 1-6 Oth of an 

 inch ; that of the rest of the syphon a 

 little larger. 



When exposed in a room, or in the 

 open air, the differential thermometer 

 remains stationary at 0, whatever may 

 be the temperature of the ambient air ; 

 but if one of its balls be more heated 

 than the other, the unequal expansion 

 of the included air puts the coloured 

 fluid in motion. In employing this in- 

 strument in experiments on the radia- 

 tion of caloric, its principal use, the ball 

 a is that to which the heat is applied ; 

 or is, as Leslie calls it, the sentient ball ; 

 and the mounting of the liquid in the 

 other stem indicates the difference, of 

 elasticity of the air in both balls, and 

 hence the name of the instrument. It 

 owes to its insensibility to general 

 changes of temperature, its peculiar fit- 

 ness for measuring the influence of ra- 

 diation. 



The theory of the instrument sup- 

 poses that gases expand uniformly with 

 equal increments of temperature : this 

 is, perhaps, not strictly true; yet, as 

 Leslie remarks, it is - so nearly correct, 

 that, in the limited range of the instru- 

 ment, the irregularity from that cause is 

 quite inappreciable. 



It is worthy of remark, that, a few 

 weeks after the publication of Mr. Les- 

 lie's " Inquiry," a part of the " Philo- 

 sophical Transactions" of London ap- 

 peared, which contained a set of expe- 

 riments almost similar to many of his, 

 and a description of an instrument, in 

 principle precisely similar to his differ- 

 ential thermometer. This was 



2. Rumfords Thermoscope. 



It consisted also of two horizontal balls, 

 united by a syphon ; and the only differ- 

 ence from Leslie's thermometer is, that 

 the scale is attached to the horizontal 

 part of the tube (which is the longest 

 portion) ; and the coloured liquid is a 

 bubble moving to and fro, when the 

 balls are unequally heated, in the hori- 

 zontal part of the tube, llumford pro- 



