230 



experimental confirmation of the helical character of the 

 magnetic wave. The energy or, as it has sometimes been 

 erroneously termed, the inertia of rotation — i.e., the resistance 

 which a rotating body oflPers to any change in the direction of 

 its axis of rotation — is well exemplified by the gyrascope, and 

 a more familiar illustration is found in the undeviating path of 

 the rifle-ball. Now, if a mass of copper be suspended by a 

 string between the poles of a powerful electro-magnet, and be 

 put in rapid rotation by twisting the string, the instant tliat 

 the magnet is excited, the rotation is arrested ; and if the mass 

 be now forcibly rotated, so much heat is developed by molecular 

 friction, that fusible metal contained in a copper tube similarly 

 placed may be actually melted and poured out. This arrest of 

 the motion of the rotating mass would be a necessary dynamical 

 sequence of the helical wave-motion assumed to constitute 

 magnetic energy ; for in that case each disturbed molecule 

 would be describing a circular orbit in a plane at right angles 

 to the lines of magnetic energy, and would by its own energy 

 resist any displacement of its axis of revolution ; and this view 

 maybe further confirmed by another experiment. A ball of copper 

 with a small pulley on its axis is placed at the end of a frame, 

 so as to be capable of being rapidly rotated by a wheel and band, 

 when placed between the poles of the electro-magnet ; its axis 

 of rotation either coinciding with, or being placed at right 

 angles to, the lines of magnetic energy. When the axis of 

 rotation of the ball coincides with the magnetic lines, there 

 would be obviously no change in the direction of the planes of 

 the assumed molecular revolution, and consequently no heat 

 ought to be developed ; this may be shown to be the case by 

 means of a thermopile connected with a galvanometer and 

 brought near the rotating ball. When, however, the axis of 

 rotation of the ball is placed at right angles to the magnetic 

 lines, heat will be immediately developed. 



36. The gyratory nature of the magnetic wave is further con- 

 firmed by a fact first observed by Faraday — namely, that if a 

 beam of polarised light be transmitted through a piece of heavy 

 glass placed between the poles of an electro-magnet, so that 

 the axis of the beam may correspond with the lines of magnetic 

 energy, then, if the magnet be excited, the plane of polarisa- 

 tion is twisted a little, either to the right or left, according 

 to the direction of magnetic polarity — a result by no 

 means inconsistent with the liypothesis of molecular revolu- 

 tion. 



37. In both the essays before alluded to, the Ether theory 

 has been put forward as evidence of the divergence of opinion 

 existing between pliysicists. It must, however, be observed that 



