Mapping the Magnetic Field. 375 



For obtaining the field of a bar magnet, the magnet was 

 placed upon a large card, previously marked with concentric 

 circles and parallel lines to facilitate the centering of the mag- 

 net. The photographic plate was placed, film side up, directly 

 upon the magnet. The iron filings were held at a distance of 

 a foot above the plate in a bag of loosely woven flannel, from 

 which, by shaking the bag, they could be evenly distributed 

 over the surface of the plate The plate was then held at two 

 points and tapped gently until the filings had arranged them- 

 selves along the lines of force. The room was, of course, up 

 to this point illuminated only by non-actinic light. The ex- 

 posure was now made by turning the key of an incandescent 

 lamp, supported at a convenient distance directly above the 

 center of the plate. The position of the lamp is a matter of 

 considerable importance, as the light from a gas jet at one side 

 of the plate will cause the filings to cast a shadow and give an 

 incorrect representation of the lines of force. Where an 

 electric lamp is not convenient, nearly as good results may be 

 obtained by holding a lighted match above the plate. It is 

 more difficult, however, to time the exposure correctly by this 

 method, and the match will often so cast a shadow upon the 

 plate as to cause irregularity of illumination. After exposure, 

 the plate is tipped so that the filings may slip off, and the few 

 which still cling to the surface of the film are carefully re- 

 moved by means of a fine, camels-hair brush. The negative 

 is then developed and fixed in the usual way. 



The plates employed should be of a sort to give the strongest 

 possible contrasts. A slow plate such as Carbutt's process 

 plate was found to give most satisfactory results. This plate 

 required an exposure of twenty seconds, when a six teen-candle 

 power lamp was placed at a distance of eighteen inches from 

 the plate. If a match is employed to illuminate the plate, 

 Carbutt's B 16 may be used, with an exposure of about three 

 seconds, and nearly as good results will be obtained. 



For making prints from these negatives, any paper may be 

 employed which will give good black and white tones. I have 

 found the aristotype paper to give beautiful results when 

 printed deeply and toned to exactly the right point Reduced 

 positives from these negatives make very brilliant lantern 

 slides, by means of which a great variety of magnetic fields 

 may be shown to a large audience. The want of such lantern 

 slides for illustrating popular lectures has been felt by the 

 writer and doubtless by many others. 



Figures 2 to 5 show typical forms of the field of a bar mag- 

 net. To obtain the best results, the bar magnet should be a 

 strong one and of considerable size, since the film is of neces- 

 sity placed the thickness of the glass plate away from the 



