636 REPORT— 1895. 



other only a few times a second, any sliift of the bands would not be capable of 

 detection by the observer. The most satisfactory results could be obtained by 

 using a constant current from a battery of a large number of cells ; but as these 

 were not at our command, we attempted to obtain a prolonp;ed discharge from a 

 battery of ten one gallon Leyden jars, a piece of wet string being included in the 

 circuit. The jars were charged by means of the above-mentioned IfuhmkorfF coil, 

 suitable arrangements being made for the purpose ; and the discharges occasionally 

 succeeded each other so quickly that for some seconds the phenomena were ap- 

 parently continuous. The duration of each discharge was determined by means of 

 Mr. Boys's wheel of lenses, and was found to exceed -^ second. Not the slightest 

 flicker of the bands could, however, be detected. 



A check experiment was then performed by dropping a piece of plate glass, 

 which would produce a shift of the bands when placed in the path of the light, 

 from such a height that its effect would last ^ second and xJ o second respectively. 

 In each of these cases a flicker was observed. 



' From this we conclude that the kathode rays and the positive column, either 

 alone or conjointly, do not affect the velocity of light passing along their path. 



8. Oil the Hysteresis of Iron in an Alternating Magnetic Field. 

 By Francis G. Baily, M.A. 



The law connecting hysteresis and induction when the latter reaches a high 

 value has not until now been ascertained. It has hitherto been assumed that the 

 hysteresis would increase continuously with the induction without limit. It is, 

 however, probable that in a slowly performed cycle of change of magnetisation a 

 maximum value of the hysteresis will be reached when the iron is saturated, and 

 that further increase of the number of lines of force induced will produce no fur- 

 ther increase in the hysteresis. When a rapidly alternating field is considered the 

 conditions are somewhat ditl'erent. As the magnetising current is increased the 

 iron reaches its saturation value at an earlier point in each peviod, thus increasiug 

 the rate of change of magnetisation. But it has been shown bj' different experi- 

 menters that the value of the hysteresis per cycle is practically unchanged through 

 wide variations in speed, and hence it may be expected that the hysteresis of iron 

 under all conditions will arrive at a definite maximum. 



To verify this experimentally, the hysteresis in a small sample of iron was 

 measured, when it was placed between the poles of a powerful electromagnet ex- 

 cited by an alternating current. Both magnet and sample were laminated, the 

 subdivision of the latter being especially fine, in order to eliminate as far as pos- 

 sible errors due to eddy currents. The sheets consisted of soft charcoal-iron of 

 thickness 'OOSS cm., and between each was a layer of tissue paper. The maxi- 

 mum value of the eddy currents was less than 2 per cent, of the h3'steresis. The 

 hysteresis was measured by the rise in temperature of the iron after 90 seconds, 

 the speed of alternation being constant at 103 cycles per second. The sides of the 

 sample were coated with layers of sheet cork, the radiation being very small. At 

 the end sufficient protection could not be allowed owing to the small size of the 

 air-gap, and hence transference of heat was prevented by maintaining equality of 

 temperature between the pole pieces and the sample by means of streams of hot 

 or cold paraffin oil over the pole pieces. All temperatures were measured by 

 thermoelectric couples of german silver and copper. 



The experiments show that the curve when plotted with the induction as abscissa, 

 and the hysteresis as ordinate, exhibits a flexure at an induction of about 16,000, 

 and becomes practically horizontal at 23,000. This corresponds to a value of in- 

 tensity of magnetisation of 1,640, which is just the saturation value. 



The same characteristics are observed when the intensity of magnetisation is 

 taken as the abscissa, the curve bending over until it is almost horizontal at the 

 point of saturation. 



The experiments prove that the hysteresis of iron is a function, not of indue- 



