(Reproduced from "Zero to Eighty" pp.315-317 by EF Northrup)

To obtain the maximum thrust in discharging a projectile
from an electric gun, the circular resistance and the
reactance of the induced current paths in the projectile
must be substantially equal. However, due to the slip —
the difference between the speed of the projectile and that of the
travelling magnetic waves which move it — the circular reactance
of the induced current paths will take different values as the
projectile gains velocity in the gun. This change of reactance is
unavoidable, so the maximum thrust cannot be continuously maintained.
Nevertheless, by having the resistance and reactance factors
reach equality at maximum velocity comparatively little is lost in
the way of thrust. Since the reactance is greatly affected by the
wave speed, the slip, and the diameter of the projectile, the
resistance factor must be adjusted to meet these conditions. By
proper selection of materials and wall thickness, the shell of the
projectile can be made to strike a reasonable compromise among
the factors of weight, mechanical strength, and resistance.

When the diameter of the projectile
is large or the frequency is
high, the construction of the projectile shell tends toward thinner
walls and alloys of high specific resistivity. On the other hand,
low frequency and small diameter indicate thicker walls and high
conductivity. Where large diameter and high frequency are
combined, it becomes a problem to have high enough resistance
without seriously sacrificing mechanical strength. One means of
overcoming this difficulty is to slot the cylinder in such a way as to interrupt the normal paths
of the induced currents and cause them
to flow in tortuous paths. When handled properly, this device of
slotting has a great effect on the resistance of the current paths
without greatly changing the reactance. Moreover, even with
considerable slotting, a cylinder will retain a great part of its original
strength and rigidity.

An aluminum cylinder slotted for experimental purposes is
shown in Fig. 10 below.

Where slotting is impractical and the projectile wall is extremely
thin, the necessary mechanical strength can be obtained by having
a supporting cylinder made of a nonconductor such as bakelite or
fibre.