LINES OF MAGNETIC FORCE — AS DELINEATED BY IRON FILINGS. 
157 
is laid over it should be level. The paper should be without any cockle or bend, and 
perfectly flat, that the filings may be free to assume the position which the magnet 
tends to give them. I have found well-made cartridge or thin drawing-paper good 
for the purpose. It should not be too smooth in ordinary cases, or the filings, when 
slightly agitated, move too freely towards the magnet. With very weak or distant 
magnets I have found silvered paper sometimes useful. The filings should be clean, 
/. e. free from much dirt or oxide ; the latter forms the lines but does not give good 
delineations. The filings should be distributed over the paper by means of a sieve 
more or less fine, their quantity being partly a matter of taste. It is to be remem- 
bered, however, that the filings disturb in some degree the conditions of the mag- 
netic power where they are present, and that in the case of small magnets, as needles, 
a large proportion of them should be avoided. Large and also fine filings are equally 
useful in turn, when the object is to preserve the forms obtained. For the distribu- 
tion of the latter it is better to use a fine sieve with the ordinary filings than to sepa- 
rate the filings first: a better distribution on the paper is obtained. The filings 
being sifted evenly on the paper, the latter should be tapped very lightly by a small 
piece of wood, as a pen-holder ; the taps being applied wherever the particles are 
not sufficiently arranged. The taps must be perpendicularly downwards, not ob- 
liquely, so that the particles, whilst they have the liberty of motion, for an instant are 
not driven out of their places, and the paper should be held down firmly at one 
corner, so as not to shift right or left ; the lines are instantly formed, especially with 
fine filings. 
3236. The designs thus obtained may be fixed in the following manner, and then 
form very valuable records of the disposition of the forces in any given case. By 
turning up two corners of the paper on which the filings rest, they may be used as 
handles to raise the paper upwards from the magnet, to be deposited on a flat board 
or other plane surface. A solution of one part of gum in three or four of water 
having been prepared, a coat of this is to be applied equably by a broad camel-hair 
pencil, to a piece of cartridge paper, so as to make it fairly wet, but not to float it, 
and after wafting it through the air once or twice to break the bubbles, it is to be 
laid carefully over the filings, then covered with ten or twelve folds of equable soft 
paper, a board placed over the paper, and a half-hundred weight on the board for 
thirty or forty seconds. Or else, and for large designs it is a better process, whilst 
the papers are held so that they shall not shift on each other, the hand should be 
applied so as to rub with moderate pressure over all the surface equably and in one 
direction. If, after that, the paper be taken up, all the filings will be found to ad- 
here to it with very little injury to the forms of the lines delineated ; and when dry 
they are firmly fixed. If a little solution of the red ferroprussiate of potassa and a 
small proportion of tartaric acid be added to the gum-water, a yellow tint is given to 
the paper, which is not unpleasant ; but besides that, prussian blue is formed under 
every particle of iron ; and then when the filings are purposely or otherwise dis- 
