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34 RA F5 OF POSITIVE ELECTRICITY

the'curves produced by the positive rays are used to identify
the in the discharge tube; for this purpose the primary

curves are the only ones that can be relied upon. The tests
for a primary line are (i) that it is parabolic, (2) that it shows
an abrupt increase in intensity at a point in the same vertical
line as the heads of the other parabolas. The first condition
is theoretically sufficient, but when only short arcs are avail-
able, it is often difficult, unless a very high degree of accuracy is
obtained In the measurement of these lines, to tell whether the
curve is or Is not a parabola.

A very interesting feature about these secondary lines is
that the velocity of the particles which produce them is practi-
cally independent of the strength of the electric field in the
discharge tube. When, as a result of a change in pressure, the
potential difference between the anode and cathode increases,
the velocity of the particles which produce the primary curves
increases also. The speed of the particles which give rise to the
secondary curves on the other hand is little if at all affected.
A little consideration will show, however, that this is what
we might expect from the theory given on page 28. The
secondaries are supposed to be due to particles which have
gained or lost a charge whilst passing through the electric and
magnetic fields. Let us consider firstly the case of the particles
which have gained a charge : they must have done so by com-
ing into collision when moving at a high speed with a corpuscle
which, as it is in the space behind the cathode where there is
no electric field, will be approximately at rest The effects of
the collision will, however, clearly depend only on the relative
motion of the particle and corpuscle, and will be the same as
if the particle were at rest and the corpuscle moving with the
velocity of the particle. Now in order that a moving corpuscle
may ionize an atom or molecule against which It strikes, the
velocity of the corpuscle must exceed a certain value which



	34 RA F5 OF POSITIVE ELECTRICITY the'curves produced by the positive rays are used to identify the in the discharge tube; for this purpose the primary curves are the only ones that can be relied upon. The tests for a primary line are (i) that it is parabolic, (2) that it shows an abrupt increase in intensity at a point in the same vertical line as the heads of the other parabolas. The first condition is theoretically sufficient, but when only short arcs are avail- able, it is often difficult, unless a very high degree of accuracy is obtained In the measurement of these lines, to tell whether the curve is or Is not a parabola. A very interesting feature about these secondary lines is that the velocity of the particles which produce them is practi- cally independent of the strength of the electric field in the discharge tube. When, as a result of a change in pressure, the potential difference between the anode and cathode increases, the velocity of the particles which produce the primary curves increases also. The speed of the particles which give rise to the secondary curves on the other hand is little if at all affected. A little consideration will show, however, that this is what we might expect from the theory given on page 28. The secondaries are supposed to be due to particles which have gained or lost a charge whilst passing through the electric and magnetic fields. Let us consider firstly the case of the particles which have gained a charge : they must have done so by com- ing into collision when moving at a high speed with a corpuscle which, as it is in the space behind the cathode where there is no electric field, will be approximately at rest The effects of the collision will, however, clearly depend only on the relative motion of the particle and corpuscle, and will be the same as if the particle were at rest and the corpuscle moving with the velocity of the particle. Now in order that a moving corpuscle may ionize an atom or molecule against which It strikes, the velocity of the corpuscle must exceed a certain value which