Temperature Coefficient of the Deflection Magnet. xlv 



Mic. Div. 



Mean balance reading corrected for temperature, Jan. 15 d — 20 d , . = 821-0 



Jan. 22*— 26 d , . = 811-9 



Mean change of reading for 7 days, = — 9-8 Mic. Div. 



Mean reading, therefore, corresponding to January 28 d before adjustment, . =806-3 



Mean balance reading corrected for temperature, Jan. 29 d — Feb. 3 d , = 766-5 



Feb. 5 d — Feb. 10 d , = 759-3 



Mean change of reading for 7 days, = — 7'2 Mic. Div. 



Mean reading, therefore, corresponding to January 28 d after adjustment, . . =770-1 

 The readings after adjustment are therefore less than before adjustment, by . 36*2 



The difference of mean readings for the two days before and after adjustment, 

 and the difference for the day before and after adjustment, are each nearly =39*0 

 mic. div., whence 37*0 has been adopted as the true difference. All the observations of 

 the balance made between Dec. 31, 1843 and January 27, 1844 have been corrected 

 by - 37'0 mic. div. 



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 quantities 

 given, pages 1 to 157, are obtained thus : n being the observed reading of the needle 

 (generally negative), t that of the theraiometer, giving the temperature of the needle, 

 q the temperature coefficient in micrometer divisions =7'90, and E, the quantity in 

 the column, " Balance Corrected" 



R = 700 + q' (t - 26) + n ; 



increasing tabular values, therefore, indicate increasing vertical force. 



The Temperature Coefficients of the Deflecting, Balance, and 



Bifilar 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 xlii. and xliii., Introduction 

 1843, for the details : the mean of all the observations gave 



The correction for 1° of Fahr., g = 0-000288 



MAG. AND MET. OBS., 1844. m 



