Page:EB1922 - Volume 32.djvu/1061

Rh The third type of high frequency (H.F.) alternator, called the homopolar inductor alternator, is represented by the machines of E.F.W. Alexanderson with disk rotor, and Bethenod-Latour and the Societe Francaise Radio-Elcctrique with drum rotor.

The principle of these alternators is as follows: a fixed ring- shaped frame or stator has inwardly projecting teeth of laminated iron, and a ringshaped magnetizing coil traversed by a direct excit- 1 ing current causes magnetic flux to spring across from the teeth on one side (N. poles) to the teeth on the other side (S. poles). This flux traverses the air gap. Over these teeth is a zig-zag armature winding. Between the teeth in the Alexanderson machine revolves a steel disk with teeth cut in the periphery (see fig. I and fig. la).

parallel, thus giving to the aerial 500 or 600 kilowatts of electric power. Also similar machines are employed in the large French radio station at Croix-d'Hins near Bordeaux, and are installed in the very large French radio station at St. Assise, near Melun, which began to be erected in 1921. Alexanderson alternators of 200 kilo- watt power are installed at the American naval radio station at New Brunswick, N.J. This station communicates with Stavanger radio station in Norway (3,554 m.) with Lyons (3,845 m.) and Nauen, near Berlin (3,958 m.) (see fig. l).

The Arc Generator. We must next mention the improvements made in connexion with the Duddell-Poulsen arc generator. In this appliance an electric arc is formed with a direct current

FIGS. I (left hand) and la (right hand). FIG. I, 200 kilowatt Alexanderson high frequency alternator, driven by an electric motor. FIG. la. Half section of the Alexanderson high frequency alternator ; showing the inductor disk which revolves between the stator poles.

(By permission of The Wireless Press.)

(The number of teeth in the rotor is half the number of teeth on teach side of the stator. These rotor teeth as they pass between the stator teeth decrease the reluctivity of the air gap and increase the magnetic flux passing. Hence as the rotor revolves the magnetic flux linked with the armature circuit is alternately increased and I decreased. This creates an electromotive force and a high fre- [quency current in the armature circuit. The only revolving part jof the machine is thus a well-balanced rigid steel disk. The field 'exciting coil and the armature coil are both stationary. Such i machines are called in English inductor alternators and in French I homopolar alternators.

In the Bethenod-Latour machines the inductor is in the form of a steel drum with exterior of laminated iron in which longitudinal grooves are ploughed out. The stator ring has inward radially pointing laminated iron teeth on both edges, and the magnetic iflux, leaving one set of teeth (N. poles), passes down through the drum teeth and up again into the other set of stator teeth (S. poles). The ridges on the drum serve to increase and decrease the flux through the armature wire which is wound zig-zag on both sets of stator teeth.

The peculiarity of the French machine is that the rotor or drum has many more ridges or teeth on it than the stator ring. By this hre can obtain the necessary high frequency without dangerous peripheral speed in the rotor, and yet leave plenty of space for the armature winding placed on the stator teeth.

On account of the few turns in the armature such machines Igive a small electromotive force, but this can be raised by suit- able static transformers, one secondary terminal of which is con- nected to the radiating antennae and the other to the earth plates. The aerial wire is then tuned to the frequency of the alternator and 'the necessary wave length. A point of importance is the exact regulation of the speed of the machines which must be kept con- stant to within o-l % of the normal. This is achieved by the use tof very sensitive governors which control the speed of the direct coupled electric motor which drives the alternator. The signals are made in the case of the Alexanderson machines by varying the inductance of a tertiary coil on the transformer which trans- fers the energy from the aerial to a non-radiative circuit. By Must as in the case of low frequency alternators in electric lighting ind power stations.
 * placing condensers in-series with the H.F. alternators it has been
 * found possible to run them in parallel, that is two or more together

High frequency homopolar alternators of the Bethenod-Latour type have been built by the Societe Francaise Radio-Electrique up to 220 kilowatts size for the French military radio station_at La Doua, near Lyons. To avoid loss of power by air churning these machines are enclosed in an air-tight case in which a partial vacuum is maintained. The speed of the machines is controlled by a Thury governor, and signals are made by short circuiting the armature coils in sections. Two or more machines can be run in

between a water-cooled copper electrode (the positive terminal) and a carbon negative electrode. The arc is inclosed in a chamber filled with coal-gas or kerosene or alcohol vapour, and a powerful transverse magnetic field is made across it (see fig. 2). If the

FIG. 2. 25 kilowatt Poulsen arc generator of electric oscillations; showing the electromagnet and arrangement for dropping alcohol into the arc box. (By permission of Marconi's Wireless Tele- graph Co., Ltd.)

arc terminals are shunted by a circuit containing a condenser and an inductance coil, high frequency oscillatory currents will be set up in this latter circuit under certain conditions. Their