488 Mr. T. Gray on Measurements of the Intensity of the 



other side of the magnetometer. To ensure that this was the 

 case, the deflector was placed on the other §ide and the position 

 of the table for no deflection again observed/ The magneto- 

 meter-needle was then adjusted, by turning the levelling- 

 screws, until the two positions were coincident. A combination 

 of the methods (1) and (2) gives a ready means of testing 

 w T hether the plates g have been properly placed on the table. 



Suppose A to be east and B west of the magnetometer. 

 The deflectors were turned by means of the plate g until their 

 lengths were accurately in the magnetic east and west line, 

 and their poles so placed as to produce a deflection to the 

 same side of zero. The deflection was then read. The plates 

 g were next turned through 180° and the deflection on the 

 opposite side of zero read. The plates g were then turned back 

 to their first position, and the deflection again read. The dif- 

 ference between the mean of the first and third reading and 

 the second gives twice the deflection. The same operation was 

 then repeated with A north and B south, with A west and B 

 east, and with A south and B west. The mean of the deflec- 

 tions for the east and west positions and the mean of the 

 deflections for the north and south positions were then found, 

 and from them the mean effective length of the two deflectors 

 calculated. This length was then substituted in equations (11) 

 and (12) below, and the value of H calculated. 



Previous to the adoption of the above apparatus only one 

 deflector was used, and it was placed by hand in the positions 

 necessary for a cycle of operations similar to that above de- 

 scribed. The idea of using two deflectors placed on opposite 

 sides of the magnetometer, and the arrangement for them 

 shown in fig. 1, is due to Sir William Thomson. It has the 

 advantage of greater symmetry, it allows the deflector to be 

 placed at a greater distance from the magnetometer, and, 

 what is most important, the magnet need not be handled 

 during the experiment. 



The Oscillation Experiment consists in finding the 

 period of oscillation of the deflector when suspended with its 

 axis horizontal, and free to perform horizontal oscillations 

 under the influence of the earth's magnetic field. This expe- 

 riment gives the value of MH from the equation 



MH + ^=^V (g) 



The quantity x refers to the torsional rigidity of the suspen- 

 sion, which was practically zero in these experiments ; /ul is 

 the moment of inertia of the oscillating system, and P the 



