Determination of the Earth's Horizontal Magnetic Force. 377 



be taken for granted, q and q can be determined by observing m' at any 

 three convenient temperatures. These temperatures, as the experiment 

 is conducted at Kew Observatory, lie usually within a degree or two of 

 0°, 18°, and 36° C. Magnets, however, destined for Arctic work are 

 exposed to a temperature below 0° C, while magnets destined for 

 tropical regions are exposed to a temperature over 40° C. 



The changes of temperature are made rapidly by introducing hotter 

 or colder water into a wooden box containing the collimator magnet. 

 Changes in the moment of this magnet, accompanying observed changes 

 in the temperature of the water, are deduced from the variations in 

 azimuth of an auxiliary magnet, freely suspended at a fixed distance 

 from the deflecting magnet. In a single experiment, the cycle of 

 temperature " hot," " mean," " cold " is repeated three times, and the 

 mean of the three observations at each temperature is used in the final 

 calculation. It is customary to have two completely independent ex- 

 periments on different days, and to take the arithmetic mean of the 

 values deduced for q and q' on the two occasions. The exact times are 

 noted at which the" several readings are taken, and suitable corrections 

 are applied from the readings of the magnetic curves for variations in 

 the horizontal force and declination. 



§ 3. Induction Coefficient. — This is denoted by ft, and really means 

 the temporary change in the magnetic moment of the collimator magnet 

 due to unit change in the field (parallel to the magnet's length), it 

 being assumed that the relation between temporary moment and 

 strength of field is linear. 



The experiment* consists in observing the angles through which an 

 auxiliary magnet is deflected out of the magnetic meridian by the 

 collimator magnet, the latter being vertical, with its north pole alter- 

 nately up and down. The vertical plane through the centres of the 

 deflecting and deflected magnets is perpendicular to the latter's axis, 

 but the centres of the magnets are not in the same horizontal plane. 



The change in the inducing field being double the intensity of the 

 vertical force at Kew is nearly 0*9 C.G.S. unit. As a rule, only one 

 complete experiment is made, but this involves inverting and reinvert- 

 ing the magnet several times. Originally fx was measured in British 

 units, so that conversion into C.G.S. units was necessary in many cases. 



§ 4. Moment of Inertia. — This means the moment of inertia of the 

 magnet and all its appendages, when at a temperature of 0° C, about 

 the suspending fibre. A collimator magnet is a hollow steel cylinder, 

 about 9J cm. long and 1 cm. in external diameter, with screws cut on 

 its inner surface at both ends. The appendages consist of two small 

 cells, one holding a lens the other a glass scale, screwed into the 



* A special apparatus is employed whose description would occupy undue space. 

 The method is practically that described on pp. 151-3 of Lamont's 1 Handbuch des 

 Erdmagnetismus.' 



2 F 2 



