large uniform demand which existed for their manufactures.
Statistics are available showing the extent to which the growth
of the electrical manufacturing industry in Great Britain was
delayed. Nearly twenty years after the inception of the industry
there were only twenty-four manufacturing companies registered
in the United Kingdom, having an aggregate subscribed capital
of under £7,000,000. But in 1907 there were 292 companies
with over £42,000,000 subscribed capital. The cable and incandescent
lamp sections show that when the British manufacturers
are allowed opportunities they are not slow to take
advantage of them. The cable-making branch was established
under the more encouraging conditions of the telegraph industry,
and the lamp industry was in the early days protected by patents.
Other departments not susceptible to foreign competition on
account of freightage, such as the manufacture of storage
batteries and rolling stock, are also fairly prosperous. In
departments where special circumstances offer a prospect of
success, the technical skill, commercial enterprise and general
efficiency of British manufacturers manifest themselves by
positive progress and not merely by the continuance of a struggle
against adverse conditions. The normal posture of the British
manufacturer of electrical machinery has been described as one
of desperate defence of his home trade; that of the foreign
manufacturer as one of vigorous attack upon British and other
open markets. In considering the position of English manufacturers
as compared with their foreign rivals, some regard
should be had to the patent laws. One condition of a grant
of a patent in most foreign countries is that the patent shall
be worked in those countries within a specified period. But a
foreign inventor was until 1907 able to secure patent protection
in Great Britain without any obligation to manufacture there.
The effect of this was to encourage the manufacture of patented
apparatus in foreign countries, and to stimulate their exportation
to Great Britain in competition with British products. With
regard to the electrochemical industry the progress which has
been achieved by other nations, notably Germany, is very
marvellous by comparison with the advance made by England,
but to state the reasons why this industry has had such extraordinary
development in Germany, notwithstanding that many
of the fundamental inventions were made in England, would
require a statement of the marked differences in the methods
by which industrial progress is promoted in the two countries.
There has been very little solidarity among those interested in the commercial development of electricity, and except for the discussion of scientific subjects there has been very little organization with the object of protecting and promoting common interests. (E. Ga.)
ELECTRIC WAVES. § 1. Clerk Maxwell proved that on his
theory electromagnetic disturbances are propagated as a wave
motion through the dielectric, while Lord Kelvin in 1853 (Phil.
Mag. [4] 5, p. 393) proved from electromagnetic theory that the
discharge of a condenser is oscillatory, a result which Feddersen
(Pogg. Ann. 103, p. 69, &c.) verified by a beautiful series of
experiments. The oscillating discharge of a condenser had been
inferred by Henry as long ago as 1842 from his experiments on
the magnetization produced in needles by the discharge of a
condenser. From these two results it follows that electric waves
must be passing through the dielectric surrounding a condenser
in the act of discharging, but it was not until 1887 that the
existence of such waves was demonstrated by direct experiment.
This great step was made by Hertz (Wied. Ann. 34, pp. 155,
551, 609; Ausbreitung der elektrischen Kraft, Leipzig, 1892),
whose experiments on this subject form one of the greatest
contributions ever made to experimental physics. The difficulty
which had stood in the way of the observations of these waves
was the absence of any method of detecting electrical and
magnetic forces, reversed some millions of times per second, and
only lasting for an exceedingly short time. This was removed
by Hertz, who showed that such forces would produce small
sparks between pieces of metal very nearly in contact, and that
these sparks were sufficiently regular to be used to detect electric
waves and to investigate their properties. Other and more
delicate methods have subsequently been discovered, but the
results obtained by Hertz with his detector were of such signal
importance, that we shall begin our account of experiments on
these waves by a description of some of Hertz’s more fundamental
experiments.
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| Fig. 1. |
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| Fig. 2. |
To produce the waves Hertz used two forms of vibrator. The first is represented in fig. 1. A and B are two zinc plates about 40 cm. square; to these brass rods, C, D, each about 30 cm. long, are soldered, terminating in brass balls E and F. To get good results it is necessary that these balls should be very brightly polished, and as they get roughened by the sparks which pass between them it is necessary to repolish them at short intervals; they should be shaded from light and from sparks, or other source of ultra-violet light. In order to excite the waves, C and D are connected to the two poles of an induction coil; sparks cross the air-gap which becomes a conductor, and the charges on the plates oscillate backwards and forwards like the charges on the coatings of a Leyden jar when it is short-circuited. The object of polishing the balls and screening off light is to get a sudden and sharp discharge; if the balls are rough there will be sharp points from which the charge will gradually leak, and the discharge will not be abrupt enough to start electrical vibrations, as these have an exceedingly short period. From the open form of this vibrator we should expect the radiation to be very large and the rate of decay of the amplitude very rapid. Bjerknes (Wied. Ann. 44, p. 74) found that the amplitude fell to 1/e of the original value, after a time 4T where T was the period of the electrical vibrations. Thus after a few vibrations the amplitude becomes inappreciable. To detect the waves produced by this vibrator Hertz used a piece of copper wire bent into a circle, the ends being furnished with two balls, or a ball and a point connected by a screw, so that the distance between them admitted of very fine adjustment. The radius of the them admitted of very fine adjustment. The radius of the circle for use with the vibrator just described was 35 cm., and was so chosen that the free period of the detector might be the same as that of the vibrator, and the effects in it increased by resonance. It is evident, however, that with a primary system as greatly damped as the vibrator used by Hertz, we could not expect very marked resonance effects, and as a matter of fact the accurate timing of vibrator and detector in this case is not very important. With electrical vibrators which can maintain a large number of vibrations, resonance effects are very striking, as is beautifully shown by the following experiment due to Lodge (Nature, 41, p. 368), whose researches have greatly advanced our knowledge of electric waves. A and C (fig. 2) are two Leyden jars, whose inner and outer coatings are connected by wires, B and D, bent so as to include a considerable area. There is an air-break in the circuit connecting the inside and outside of one of the jars, A, and electrical oscillations are started in A by joining the inside and outside with the terminals of a coil or electrical machine. The circuit in the jar C is provided
