374 C. B. Thwing — Photographic Method of 



Aet. XLY. — A Photographic Method of Mapping the Mag- 

 netic Field ; by Chakles B. Thwing. 



The common method of obtaining a cross section of the 

 field of force of a magnet by means of iron filings is very 

 satisfactory when only temporary representations of the field 

 are desired. There has been wanting hitherto a suitable 

 method of making a permanent map of the magnetic field. 



A method employed in the physical laboratory at North- 

 western University has been found to give results so far 

 superior to any hitherto obtained, that a description of the 

 method employed will probably prove of value to others who 

 may be engaged in the study of the magnetic field. 



While studying the magnetic field, it was suggested by Mr. 

 John Lam ay that a permanent record of the field could be 

 obtained by securing a map upon blue print paper, exposing to 

 the sun, and washing as usual. The results obtained were 

 beautiful ; but it was of course necessary to repeat the process 

 for every copy of the field desired. It at once occurred to the 

 writer that by employing a photographic dry plate, in place of 

 the blue print paper, a negative would be obtained from which 

 any desired number of prints could be made and which would 

 have the additional advantage of representing the figures in 

 their true colors. 



Fig.l. Apparatus employed. 



The apparatus employed is shown in fig. 1, which also illus- 

 trates the method used in distributing the iron filings and 

 exposing the plate. 



