xliv Introduction to the Makerstoun Observations, 1845 and 1846. 



Bijilar Magnet. 



Qb. The temperature coefficient for this magnet was also determined by means 

 of hot and cold water experiments, Nov. 9 and 10, 1843. See page xli.. Introduc- 

 tion, 1841-2, for the details. The whole number of results was 30 : the mean gave 



The correction for 1° Fahr., Q = 0000294. 



66. If the 27th and 28th results (counting from the top of the last column of 

 Table 19, p. xli., Introduction, 1841-2) be rejected, as it is believed that the great 

 difference of both from the mean was probably due to one error in reading, we find 

 the average difference of the 28 results from the mean = 0*000021, and the probable 

 error of a single result was therefore about 0*000017. Combininnj the results from 

 high temperatures together, and similarly for those from low temperatures, we find 



The mean of 15 results for the mean temperature 48°-0 gave Q = 0000292 

 The mean of 13 68°-7 Q = 0-000295 



so that for the bifilar magnet, also, the temperature coefficient is constant within the 

 ordinary temperatures of 32° to 80° Fahr. 



67. The correction for the expansion of the silver wires and brass grooved 

 wheel, =0*000010, being added to the value of Q above, we have 



The temperature correction for 1° ¥ahr.,from hot and cold water experiments, q = 0"000304. 



68. As the observations in connection with the balance needle had shewn that 

 there might exist variations due to temperature, other than those due to the varia- 

 tion of the magnetic moment of the magnet, such as the varying elasticity of the 

 suspending wire of the bifilar magnet, the temperature coefficient was determined 

 in the following manner, which had at first been found to give consistent results for 

 the balance needle. 



69. A series of days being selected in which the magnetic irregularities are 

 small, and in which the variations of temperature are as considerable as possible, if 

 we compare the mean instrumental readings for any two days, and if a R be the 

 difference in scale divisions, this difference is due to change of temperature of the 

 magnet, and to change of the horizontal component of the earth's magnetism, let the 

 portion of change of reading due to the former = A, and to the latter = a X, so that 



A R = A + A X. 



If the difference of the mean temperatures of the magnet for the same two days 

 be A i, then the correction for 1° of temperature in scale divisions 



