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i8 RAYS OF POSITIVE ELECTRICITY

A depends merely upon the strength of the magnetic field
and the distance from the point of projection at which the
deflection is measured ; it is quite independent of the charge,
mass, or velocity of the particle.

If the magnetic field is that between two poles of an
electromagnet placed close together and reaching up to the
point of projection of the particle, then if a is the breadth of
the pole pieces, H is approximately constant from #=<? to
x=a and vanishes from x=a to #=/. Substituting this value
for H in the expression for A we find

—('-3

when H is the magnetic force between the poles. When this
approximation is not sufficiently accurate and we have to
take into account the stray magnetic field beyond the poles
as well as the variation of the magnetic force between the
poles, A may be conveniently determined by the following
method.¹ Wind a coil of triangular section DEF, the base
DF being equal to /, the angle EDF a right angle, and DE

small compared with the depth of the pole pieces of the
electromagnet. Place the coil so that DF is along the direc-
tion in which the particle is projected, D being at the
point of projection and F at the distance at which the
deflection is measured, connect up the coil with a ballistic
¹ J. J. Thomson, " Phil. Mag.," VI, xvili, p. 844.



	i8 RAYS OF POSITIVE ELECTRICITY A depends merely upon the strength of the magnetic field and the distance from the point of projection at which the deflection is measured ; it is quite independent of the charge, mass, or velocity of the particle. If the magnetic field is that between two poles of an electromagnet placed close together and reaching up to the point of projection of the particle, then if a is the breadth of the pole pieces, H is approximately constant from #=<? to x=a and vanishes from x=a to #=/. Substituting this value for H in the expression for A we find

	—('-3	H

	when H is the magnetic force between the poles. When this approximation is not sufficiently accurate and we have to take into account the stray magnetic field beyond the poles as well as the variation of the magnetic force between the poles, A may be conveniently determined by the following method.¹ Wind a coil of triangular section DEF, the base DF being equal to /, the angle EDF a right angle, and DE



	small compared with the depth of the pole pieces of the electromagnet. Place the coil so that DF is along the direc- tion in which the particle is projected, D being at the point of projection and F at the distance at which the deflection is measured, connect up the coil with a ballistic ¹ J. J. Thomson, " Phil. Mag.," VI, xvili, p. 844.