396 Dr. C. Chree. Collimator Magnets and the 



§30. Variations of horizontal force and declination — which likewise 

 affects both vibration and deflection readings — are more serious. If 

 small and gradual, they are of course likely to be largely eliminated, 

 especially in the deflection experiment, where the order of the operations 

 is specially devised with a view to this end. If, however, they are 

 large and sudden, probably the only really satisfactory course is to 

 reject the observation. 



It might be supposed that at a fixed observatory corrections could 

 always be applied by referring to the magnetograph curves. This, 

 however, is not very feasible in practice. The time scale of the curves 

 is so contracted that the time to which a particular ordinate refers is 

 usually uncertain to the extent of at least half a minute. Again, the 

 position at any instant of a magnet during rapid changes of field 

 depends to an extent difficult to . determine on the inertia of the 

 magnet; and the magnets used in magnetographs and in absolute 

 instruments differ greatly from one another both in inertia and in 

 method of suspension. The vibration of a magnet under sudden 

 irregular variations of horizontal force and declination is in all prob- 

 ability a very complicated problem. 



§ 31. The possibility of obtaining satisfactory horizontal force 

 measurements depends largely on avoiding times of serious magnetic 

 disturbance. 



The shorter the time occupied by the observations, the better the 

 chance of accomplishing this. Supposing a uniform procedure adopted, 

 the time occupied by a vibration experiment is independent of the 

 observer ; it is shorter the stronger the magnet and the greater the 

 horizontal force. It is thus of especial importance that collimator 

 magnets intended for work in arctic regions, where the force is low 

 and disturbances large and numerous, should be of high magnetic 

 moment. 



In the deflection experiment a great deal depends on the skill of the 

 observer, and on the make of the unifilar ; one observer may do in 

 twenty minutes what occupies another for fifty. Extreme conscien- 

 tiousness may be a positive demerit, owing to the excessive time spent 

 in adjustments and readings. 



A variety of other criticisms will be considered which apply to some 

 one physical quantity, and are classified accordingly. 



§ 32. Moment of inertia. — In default of any simple means of directly 

 measuring the variation with temperature of the moment of inertia, 

 this variation is calculated by assuming for the coefficients of linear 

 expansion : — 



18 x 10 ~ 6 in the auxiliary brass bar, 

 12 x 10~ 6 „ „ collimator magnet. 

 There is of course the uncertainty that the assumed mean values may 



