1898-99.] Mr David Robertson on Lines of Force. 
361 
Dust Figures of Electrostatic Lines of Force. By- 
David Robertson, A.G.T.C. Communicated by Professor 
Jamieson, M.Inst. C.E., of the Electrical Engineering Depart- 
ment of the Glasgow and West of Scotland Technical College. 
(Read December 19, 1898.) 
When lecturing on Electrostatics at the Glasgow and West of 
Scotland Technical College, the author endeavoured to impress 
upon his students the identity of the laws of force for magnetism 
and electricity at rest. He thought it would further this idea if he 
could get pictures of electrostatic lines analogous to those obtained 
from magnets by means of iron filings, and an analysis of the 
difference of the conditions in the two cases led to the conclusion 
that there was no reason why this should not be done. 
When we shower iron filings upon a glass plate, or a sheet of 
paper, under which a magnet is placed, unlike poles are induced at 
the ends of each filing, and it becomes for the time being a little 
•compass needle. Since it is very short, the total action upon it 
will be a couple turning it into the direction of the force at its 
centre, unless it be close to the magnet, in which case there will be, 
in addition, a translation along a line of force. This latter effect 
■causes the filings to crowd towards the edges of the magnet. 
How, in electrostatics we also have an inductive action, opposite 
charges being induced in the near and far ends of each particle as 
it comes into the field, but we have the important difference, that 
whereas the magnetic poles are attached to the filing and cannot 
be separated from it, the electric charges will leave a body if they 
have an opportunity of doing so. To avoid this difficulty as far 
as possible, the substance we employ should have some insulat- 
ing qualities to prevent the immediate escape of the charges, and 
yet should be a sufficiently good conductor to allow enough elec- 
trical separation to take place. Further, each particle should be 
greater in one dimension than in the other two, in order that there 
may be a tendency to bring it into a definite direction. 
VOL. XXII. 11/2/99. 2 A 
