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42 AMATEUR TALKING PICTURES
takes place by the needle entering the groove and resting on its bottom, as will be gathered by the dotted lines shown in the figures.
Looking now at B we find in the left-hand figure the approximate dimensions of the R.C.A. Victor pre-groove record. This should be compared with the dimensions of the standard gramophone record as already given in Fig. 6.
The needle employed for recording has a very obtuse point, as shown in illustration C, in fact, examining it casually one would wonder how it could possibly be used for recording or playback, since it is totally unlike the ordinary gramophone needle.
Actually when the point of this needle acts on the groove considerable pressure is exerted on the edges which owing to the rotation of the disc are embossed or pressed down something after the style shown in the illustration at the right of B. Thus it is that recording takes place on the upper edges of the groove, and not the bottom which remains a true spiral. The action will be gathered by examining the illustration D taken across the section of recorded groove showing exactly how recording is effected.
Looking at the matter from a theoretical aspect, it would be imagined that the process is by no means so perfect as where cutting or scribing is effected straight into a plain disc. In the experience of the writer, however, the pregrooved R.C.A. Victor discs give, in general, considerably better results than those of plain aluminium. Furthermore, they possess another advantage in that the needle used for recording is also employed for playback. It is not so much the fact that the same needle is used as that it is of metal. Fibre needles, athough they give pleasing reproduction, are apt to break away or wear at the point under service. Although this may not be very disastrous from the point of view of ordinary recording when we are dealing with talking pictures on the disc system as described in Chapter IX, the results are fatal.
The Pacent Recordovox. The Pacent Recordovox is one of the best known equipments for utilizing the pre -grooved disc. The illustration in Fig. 22 shows that the apparatus is simple, and consists of a standard type of Pacent pick-up