156 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXIX.) 
bar preserve their full state of magnetism ; i. e. the sum of lines of force in the equator 
of either of the new magnets is equal to the sum of lines of force in the equator of the 
original unbroken bar. I took a steel bar 12 inches long, 1 inch broad and 0*05 of an 
inch thick, made it very hard, and magnetized it to saturation by the use of soft iron 
cores and a helix ; its power was 6°*9. I broke it into two pieces nearly in the middle, 
and found the power of these respectively 5°*94 and 5°'89; indicating a fall not more 
than was to be expected considering the saturated state of the original magnet. When 
these halves were placed side by side, with like poles together as a compound mag- 
net, they had a joint power of 11°*06, which, though it shows a mutual quelling in- 
fluence, is not much below the sum of their powers ascertained separately. All this 
is in perfect harmony with the voltaic battery, where lines of dynamic electric force 
are concerned. If, as is well known, we separate a battery of 20 pair of plates into 
two batteries of 10 pair, or 4 batteries of 5 pair, each of the smaller batteries can 
supply as much dynamic electricity as the original battery, provided no sensible 
obstruction be thrown into the course of the lines, i. e. the patli of the current. 
3233. When magnets are placed in an adverse position, as neither could add 
power to the other in the former case, so now each retains its own power ; and the 
lines of magnetic force represent this condition accurately. Two magnets placed end 
to end with like poles together are in this relation ; so also are they if placed with 
like poles together side by side. In the latter case the two acting as one compound 
magnet, give a system of lines of force equal to the sum of the two separately (3232.), 
minus the portion which, as in imperfect magnets, is either directed inwards by the 
softer parts or ceases to be excited altogether. 
§ 37 . Delineation of Lines of Magnetic Force hy iron filings. 
3234. It would be a voluntary and unnecessary abandonment of most valuable aid, 
if an experimentalist, who chooses to consider magnetic power as represented by lines 
of magnetic force, were to deny himself the use of iron filings. By their employment 
he may make many conditions of the power, even in complicated cases, visible to the 
eye at once; may trace the varying direction of the lines of force and determine the 
relative polarity ; may observe in which direction the power is increasing or dimi- 
nishing ; and in complex systems may determine the neutral points or places where 
there is neither polarity nor power, even when they occur in the midst of powerful 
magnets. By their use probable results may be seen at once, and many a valuable 
suggestion gained for future leading experiments. 
3235. Nothing is simpler than to lay a magnet upon a table, place a flat piece of 
paper over it, and then, sprinkling iron filings on the paper, to observe the forms they 
assume. Nevertheless, to obtain the best and most generally useful results, a few 
particular instructions may be desirable. The table on which the magnet is laid 
should be quite horizontal and steady. Means should be taken, by the use of thin 
boards or laths, or otherwise, to block up round the magnet, so that the paper which 
