1910.] 



Effect of an Altematincj Magnetic Field. 



397 



results in Lord Crawford's experiments, I have further explored the matter, 

 and have found a means of producing the visual effect (which is physiological 

 and subjective) in a way that succeeds with every person on whom it has yet 

 been tried. 



An alternating magnetic field of sufficient intensity and extent was 

 produced by passing an alternating electric current around a specially 

 constructed magnetising coil. This coil, consisting of 32 turns of stranded 

 copper wire, having an equivalent cross-section of about 0"2 square inch in 

 each turn, was wound upon a wooden cylinder about 9 inches in diameter, 

 and about 8 inches in length; the cylinder being then removed. The 

 alternating current, of a frequency of 50 periods per second, could be 

 increased up to 180 amperes at will ; so that the total number of ampere- 

 turns reached 5760. The intensity of the alternating magnetic field at the 

 centre of this coil had, therefore, a virtual value (quadratic mean) of about 

 1000 C.G.S. units ; and its instantaneous maximum value (at the centre of 

 the coil) was therefore about 1400 C.G.S. units. The value at the mouth of 

 the coil was not much more than two-thirds of this amount, and was not 

 uniform over the cross-section of the interior space. 



On inserting the head into the interior of the coil, in the dark, or with the 

 eyes closed, there is perceived over the whole region of vision a faint 

 flickering illumination, colourless, or of a slightly bluish tint. The period of 

 the flicker is not well defined. It does not seem to be the same over the 

 whole region of vision at the same time, nor is it eq\ially bright over the 

 whole region of vision, but is somewhat brighter in the peripheral region than 

 in the central parts. Even in daylight, with the eyes open, one is conscious 

 of a sensation of flicker superposed upon the ordinary vision. 



The effect is diminished by lowering the intensity of the field, and increased 

 by raising it. Attempts to discover whether the brightness of the pheno- 

 menon stands in any relation to the direction of the axis of the field with 

 respect to the principal axes of the skull have not yet revealed any definite 

 result. It will be necessary to apply more intense fields than have yet been 

 tried. No after-effects of any kind have been experienced either by myself 

 or by any of the persons who have made the experiments with me. 



As yet it has not been possible to ascertain whether varying the frequency 

 has any effect on the phenomenon. 



The phenomenon is so distinct, when once it has been seen, that it is 

 difficult to believe that it has not been observed before by those who have 

 been working with transformers. Nearly four years ago Prof. Birkeland, of 

 Christiania, told me that his workmen at the nitrate factory at Notodden 

 declared they could see lights over the choking coils used to limit the 



