– 41 –
Then see 2x10 om %o
te spree 032
a ity = 6.95
The situation is thus entirely satisfactory. The carrier frequency could even be halved.
(viii) The choice of medium. In choosing the medium we have to take into account
(a) That a medium with a small characteristic impedance such as water has a slight advantage as regards the factor R(w).
(b) That water is more attenuative than mercury.
(c) That mercury gives wide band widths more easily than water because of closer matching, but that adequate band widths are nevertheless possible with water.
(d) That a water-alcohol mixture can be made to have a zero temperature coefficient of velocity at ordinary temperatures.
On the whole the advantages seem to be slightly on the side of mercury.
(ix) Long lines.— The idea of using delay lines with a long delay, e.g. of the order of 0.1 second, is attractive because of the very large storage capacity that such a line would have. Although the long delay would make these unsuitable for general purposes they would be very suitable for cases where very large amounts of information were to be stored: in the majority of such cases the material is used in a fairly definite order and the long delay does not matter.
However such long lines do not really seem to be very hopeful. In order to reduce the attenuation to reasonable proportions it would be necessary to abandon carrier working, or else to use mercury. In either case we should probably be obliged to make the tank in the form of a bath rather than a tube; in the former case in order to avoid the phase distortion arising from reflections from the walls, and in the latter to economise mercury, using a system of mirrors in the bath. In any case the technique would involve much development work.
We propose therefore to use only tanks with a delay of 1 ms.
(x) Choice of parameters.— Considerations affecting the carrier frequency are:
(a) The higher the carrier frequency the greater the possible band width.
(b) The difficulty of cutting thin crystals, somewhat modified by the absence of necessity of frequency stability.
