TEMPERATURE COEFFICIENT OF THE DEFLECTION MAGNET. xl 
The observations after July 24 are reduced to those before that date by the 
above quantity. 
62. The observations of the balance magnetometer are made in the following 
manner :—The moveable wire of the right micrometer is made to bisect the spider- 
cross half the time of vibration in the vertical plane before the minute of observation, 
and that of the left micrometer as long after the minute; the mean of the two read- 
ings gives the position of the needle at the minute. The readings increase posi- 
tively when the north pole of the needle moves below the horizontal. The tabular 
observations given, in this column, are obtained thus: n being the observed reading 
of the needle (generally negative), ¢ that of the thermometer, giving the temperature 
of the needle, g’ the temperature coefficient in micrometer divisions = 7-90, and R 
the quantity in the column, “ Balance Corrected” 
Jan. 1¢1845—July 141846. R= 700 + 9’ (¢— 26) +»; 
July 34 1846—Dec. 314 1846. R= 124+ 7 (¢ — 26) +0; 
increasing tabular values, therefore, indicate increasing vertical force. 
Tue TEMPERATURE COEFFICIENTS OF THE DEFLECTING, BALANCE, AND 
BiritaR MAGNETS. 
Deflecting Magnet. 
63. The temperature coefficient of the large deflecting bar (15 inches long), used 
in the observations for the absolute horizontal intensity, was determined November 
11, 1843, by hot and cold water experiments, see pages xli. and xliii., Introduction 
1843, for the details: the mean of all the observations gave 
The correction for 1° of Fahr., 7 = 0:000288 
64. The observations from which this result was obtained were very good, con- 
sidering that the whole angle of deflection was less than 3°, and it may therefore be 
worth examining the individual results for the highest and lowest temperature. The 
whole number of results was 15, the mean difference, from the final result given 
above, =0-000025, and the probable error of a single result was therefore about 
0-000021. 
The mean of 9 results for the mean temperature 49°7 gave g = 0-000289 
The mean of 6 ................- ee erate eee GS g = 0:000286 
so that the temperature coefficient for this bar is constant within the ordinary tem- 
peratures occurring during the observations in which it was employed. 
