324 Mr. T. Gray on the Experimental Determination 



of separate parcels by plunging them, after having been heated 

 to a bright red heat, into oil of various temperatures. These 

 bars were not again heated. All the bars were then magnetized 

 by the current from ten of Thomson's Tray Daniells, flowing 

 through a magnetizing coil, of the same length as the bars, 

 made of insulated copper wire. This coil consisted of four 

 layers of forty turns each, and had a resistance of *065 ohm. 

 Thus, taking the electromotive force of a Tray Daniell as 

 10 8 C.G.S. units, and its resistance as *1 ohm or 10 8 C.Gr.S. 

 units, the current was, in absolute measure, approximately, 



, --_ . This was distributed over 160 turns of a solenoid five 

 1*065 



centimetres long ; and therefore the magnetizing force was 



1-065 X T X 4?r = 377 near1 ^ 



Before the magnetic moments could be calculated, it was of 

 course necessary to determine the horizontal component of the 

 earth's magnetic force at the place where the magnetometer 

 was to stand during the experiment. This was done by 

 observing the period of oscillation, under the horizontal com- 

 ponent of the earth's force, of five separate magnets, each 

 suspended by a silk fibre about thirty centimetres long, and 

 enclosed in a glass case placed on the magnetometer-stand» 

 The magnetometer was then placed on its stand, and the 

 deflection of its needle by each of these magnets, placed 

 with its centre at a distance of twenty centimetres from the 

 needle, observed. A reading was taken with the magnet 

 resting in the groove on the arm A, which was placed in an 

 east-and-west direction. The arm was then turned through 

 180°, and a reading taken with the magnet in its new position. 



The same operations were repeated with the ends of the 

 magnet reversed, and the arithmetic mean of these four read- 

 ings taken as the deflection on the scale. 



The formulas for deducing the horizontal component and 

 the magnetic moment are easily obtained, as follows : — 



Let H = horizontal component of the earth's magnetic 



force. 



T = period of vibration of magnet under H. 



yit = moment of inertia of magnet round an axis 



through its centre at right angles to its length. 



r = distance of centre of magnet from centre of 



needle. 

 a = virtual half-length of the magnet (that is, half 



the distance beteen its poles). 

 6 = deflection of needle in degrees. 

 M as moment of magnet. 



