554 Dr. M. Hartog. [ Dec. 1, 
material into its cross section, divided by the length of the path travelled by 
the lines. 
Now the laws of distribution of the lines in a magnetic field are these :-— 
1. All lines travel from one pole to another of opposite sign, in continuous 
curves, which may, however, only be completed at infinity. 2. The number 
of lines in any part of a field is proportionate to the permeance of the path 
they have to travel. The terms “reluctivity,” “reluctance,” are defined as 
the reciprocals of permeability and permeance respectively; it is often more 
convenient to use them, and to speak of a material as of “ high reluctivity ” 
than as of “ low permeability.” 
VI. 
The path traversed by these lines in air can be approximately shown in the 
case of magnetism by placing a piece of smooth paper horizontal in the field 
to be studied, strewing it with magnetic dust, such as iron filings, reduced 
iron, or powdered magnetic oxide of iron (Fe30,)* and gently agitating 
the paper, when the powder is seen to arrange itself in the well-known curves. 
To obtain such delineation in fields with varied poles I found it necessary 
to devise a special apparatus, which has been constructed for me by the 
Crypto Works Company, Ltd. (now the Crypto Electrical Company). 
A square base-plate of cast iron supports four columns of brass, screwed at 
their upper ends, and fitted with large flat nuts. Vertical electromagnets 
are provided, with the soft iron cylindrical cores longer than the coils, and 
rounded above to give more horizontal spread to the lines leaving them: they 
rest on the base-plate, which serves to lessen the air-gap; and is, for a pair of 
straight electromagnets with opposite poles, the equivalent of the yoke or cross- 
piece of a horseshoe-magnet. The extra length of the cores is a device to 
secure stability: either the top of the core passes through a hole in a per- 
forated slab of wood supported on the brass columns; or the coil rests on a 
perforated cylinder of lead through which the lower part of the core passes, 
in which case a thin plate of brass may replace the wooden slab. The latter 
arrangement: allows any distance to be put between the poles, instead of the 
limitations imposed by the perforations of the slab. The coils are attached 
at the lower end by flexible connections (of lamp-cable) to the binding-screws 
of a mercury commutator, which allows of the ready reversal of the polarity 
of any magnet singly. An ammeter, a rheostat, and a source of constant 
current at 4 volts complete the apparatus. The coils will stand for a short 
time a current of 5 amperes as a maximum. 
* IT use the native magnetite, crushed, levigated, separated from impurities by 
magnetism, and dried ; it is much blacker than metallic iron, and the figures show up 
better. 
