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diffraction patterns can also be taken for small areas that have been
selected microscopically. The magnetic lenses have been redesigned
with the windings in air so that only the central core of the lens
has to be evacuated. In this case the high tension supply is provided
by a rectifier unit driven from a high frequency oscillator, which
enables voltage stabilization to be obtained more easily than with
a 50-c/s set. The electron microscope opens up fields of investigation
far beyond the reach of the most powerful optical instrument. It has
already become a valuable tool for the routine examination of industrial
pigments, emulsions, powders, fibres and the like and for the study
of viruses, bacteria, diatoms and other biological specimens.
 A
complementary instrument, the electron diffraction camera, was also
developed. It enables the surface conditions in metallic or non-metallic
materials to be examined by means of diffraction patterns produced
by an electron beam, which is either reflected at glancing incidence
from the surface or transmitted through thin films of the material.
This method, which is analogous to the use of x-ray diffraction
for examining internal structure, is of value in studying catalysis
and corrosion. A 100-kV camera is now in production.
OTHER INVESTIGATIONS
The range of oil diffusion pumps, on which the feasibility of continuously
evacuated apparatus depends, has been much extended in recent
years, and pumps are now available in sizes up to 32 inches diameter
and pumping speeds of 6000 litres per second at a pressure of 10-4
mm of mercury. The smaller sizes are used for high vacuum induction
furnaces and large aluminizing plants, and the larger on atomic
energy projects.
Much
work has been done on the behaviour of materials at high temperatures.
A full knowledge of strains and stresses is essential for the design
of modern turbines, particularly gas plant, and new testing methods
and equipment have been developed for the investigation of creep,
fatigue, yield and allied phenomena.
Equipment
was developed in 1945 to give a continuous indication and record
of shaft eccentricity and differential expansion between the rotor
and the casing of steam turbines; when using very high temperatures,'
any unequal heating may give rise to serious damage. Another device
will detect vibration in bearing pedestals, thus making an additional
contribution to reliability in generating plant.
Experiments
on the basic mechanism of metal cutting, directed by R. N. Arnold,
have led to important improvements in the design of cutting tools
and the choice of speeds, feeds and other factors in machining.
For instance, cutters of unconventional design have been developed
for the extremely severe conditions of machining the Nimonic heat-resisting
alloys; these may involve cutting stresses five or six times as
great as those of steel. On one operation the new cutter enabled
double the number of pieces to be machined for each tool regrind.
An improvement ofji only a few per cent in a widely used operation
can, of course, bring a considerable increase in production and
saving in cost.
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