
1904. ] The Dual Force of the Diiding Cell. 555 
The perforated slab or brass plate serves as a “stage” on which we can place 
our paper; or, if we wish to delineate our field in a liquid medium, a glass 
plate is placed on the paper, which now serves to reflect the light, better illu- 
mination being thus obtained than by the use of porcelain. To delineate the 
field with dust in air, I sprinkle the dust on the paper by shaking it through 
fine copper gauze covered by a cambric rag; and for tapping I use the “ con- 
nection ” of a bicycle inflator—consisting of two metal nuts united by thick 
rubber tube—which is convenient as enabling me to regulate the taps with 
great delicacy. It is often best to tap the base or columns of the apparatus 
rather than the stage or the paper. For the study of the “ chains ” in liquid 
I use a mixture of the magnetic dust with glycerine, dissolved or melted 
balsam, or melted jelly, spread on a glass or porcelain plate. Under the 
influence of the field the liquid soon differentiates into chains of the 
magnetic material, separated by clear spaces, and the effects of the con- 
tinuance or increase of the magnetic force can be studied. 
A Leitz “large microphotographic apparatus,” provided with a 5-inch RR 
lens, and with the camera vertical, serves to record the figures obtained; the 
magnetic apparatus being placed on the base-plate of the photographic 
Instrument. 
Vk 
We can now study the nature of the figures obtained by our apparatus, and 
we begin with the dust-paper results. While resting on the paper, the dust 
is kept stationary by friction ; but, on tapping, it rises in a cloud into the air, 
and is free to respond to the magnetic stresses present, and to arrange itself 
into the material curves which we term “chains of force,” and define as the 
“files of more permeable substance segregated along lines of force out of a 
mixture under the stresses of a centred force”: in this case the mixture is that 
of air and dust in the cloud. Faraday, indeed, already noted that the presence 
of the iron dust was a disturbing factor in the distribution of the lines of force 
in alr which he employed it to demonstrate ; but the first explicit recognition 
of such chains as a system to be studied is, I believe, to be found in the 
preliminary note cited above. 
By the laws of the distribution of lines of force, such a chain of particles, 
possessing n-times the average permeability (taking into account the air 
intervening between its particles) of an adjacent tube of air of equal section, 
will contain n-times as many unit lines of force approximately. 
The properties of these chains are far better studied when formed in a 
viscid medium, which acts as a bond between their successive particles. 
Here their toughness is remarkable. With a permanent magnet I have 
obtained horizontal spindles in glycerine in a vertical trough which, despite 
