34 Scientific Proceedings, Royal Dublin Society. 



the lines of force to the opposite pole, which statement is equivalent to defining 

 a line of force; but beyond the general statement that a permeable body- 

 would always move so as include the greatest number of lines of induction, 

 there was no indication of the actual trajectory. We therefore determined to 

 investigate the problem for ourselves as follows : — 



Descriptive. 



Our first experiments were made with circular discs of thin charcoal iron, 

 cemented by paraffin to the lower faces of discs of cork or linoleum to float 

 them. The liquid used was water, and the method of observation was to 

 powder the surface with starch or flour so as to increase the frictional 

 resistance and, at the same time, to define the passage of the floating disc by its 

 wake. 



Among our preliminary experiments we wish to note three that are well 

 suited for demonstration purposes. A Gillette razor-blade floating on the 

 surface film of water, or gum-water, shows very clearly the relation of our 

 curves to the lines of force of the field, whether homo-polar or hetero-polar. 

 Still better is a light compass needle 1 pivoted on a pin, attached to a cork 

 float ; the compass-needle is at all points tangential to a line of force, while 

 the float bearing it of course follows such a curve as we have described. 



A magnetic shell formed by punching a disk from an annealed Gillette 

 blade, hardening it and magnetizing it in a coil traversed by a strong current, 

 and then attached to a disk of cork, follows the lines of force of the field ; 

 this was devised to model the behaviour of a charged body in an electrostatic 

 field, as contrasted with the path of a body more permeable than the medium. 



This was unsuited to give accurate records of the path followed. We 

 replaced the water by various viscid liquids, of which glycerine soon showed 

 its superiority, though it has the disadvantage of becoming hydrated during 

 the time of experiment. The disturbing effects of currents and of surface 

 tension were reduced very greatly with this liquid, the latter being noticeable 

 only towards the extreme edge of the trough. 



We soon had to replace the floating discs, since, owing to their form, they 

 gave a lurch whenever the exciting current was put on or increased. This was 

 avoided by using a spherical float. Paraffin softened by gentle heat was com- 

 pressed in a bullet-mould ; the part adjoining the channel for pouring in lead 

 was melted with a hot needle, and reduced iron pushed in. The sphere 

 measured § inch in diameter, and contained about 0-13 gms. of iron. 



We attempted to record the path by covering the trough, through which 

 the magnet poles projected, with a sheet of ground glass, rendered transparent 



1 A more permeable body and a magnet of course follow similar paths, since the lines 

 of induction in the former make, it a magnet for the time being. 



