Temperature Coefficient of the Balance Magnet. xlix 



Whether the results for each year have equal weights, or have weights depend- 

 ing on the sums of differences of the daily mean temperatures (2 a t), we find 



q' = 1-93 sc. div. 



The adopted value of the temperature coefficient of the bifilar magnet, 

 q' = 1'90 sc. div. 



The value of one scale division in parts of force for the period of comparisons 

 (1844 and 1845), being k = 0-000140. 



Whence, the correction for 1° Fahr., from comparisons of observations, is q = 0-000266. 



74. The result from hot and cold water experiments is nearly I more. It ap- 

 pears, therefore, that the determination of the temperature coefficient, by removing 

 the magnet from its position in the instrument and varying its temperature by means 

 of hot and cold water, cannot be depended on. It appears also, that when a suffi- 

 cient number of observations is included, the method of comparison previously de- 

 scribed gives, under very different conditions, consistent, and, therefore, it is pro- 

 bable, accurate results.* 



Balance Magnet. 



75. The temperature coefficient of the balance magnet was determined by means 

 of hot and cold water experiments August 24, September 1 and 2, and November 

 13, 1843, and January 27, 1844. See pages xlii., xliii., and xliv., Introduction, 1841-2, 

 for the details. The mean of the whole observations, properly weighted, gave 



q = 0-000073. 



76. The only good series was that obtained January 27, 1844, which included 

 changes of temperature from 35° to 65° only ; the other series are too inaccurate to 

 be employed for the determination of the value of q for high and low temperatures ; 

 from series of comparisons of the usual observations of the balance it has been 

 found, however, that the value of q\ the temperature correction for 1° Fahr. in mi- 

 crometer divisions, is the same for high and low temperatures, thus — 



Mic. Div. 



From 7 series of comparisons in 1844 and 1845, about the mean temperature 40°, ^=8-33 

 From 8 60°, ^ = 8-30 



As the first result is the mean of 7 values of q, obtained from comparisons of 

 the mean readings of the balance magnetometer for about 170 days, in the months 

 of January, February, November, and December 1844 and 1845 ; and as the second 



* It should be remarked, that these conclusions do not depend wholly upon the results for the 

 Makerstoun instruments, their accuracy has been verified by an examination of the observations made 

 in other places. 



MAG. AND MET. OBS., 1844. n 



