contents - History of Ericsson - History of Ericsson

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For a time gelatine appeared to meet all requirements. With a sharply ground
engraving needle, set at an angle of 75° to the disc, there was obtained a thickcohesive
shaving. The frequency range was satisfactory, but in view of the
comparatively coarse structure of the gelatine the needle scratch was rather
serious. Moreover the gelatine was highly susceptible to moisture and heat.
These dises were relatively cheap and are still used with satisfactory results
by amateur beginners. The reproducing is done with curved steel needles and
to minimise friction between needle and record vaseline is rubbed into the
disc after recording.
In 1931, however, Svenska Radioaktiebolaget began to experiment with recording
discs of acetate and cellulose lacquer. Great difficulties were, however,
encountered in successfully experimenting with the recording material,
as it was only gradually that the chemical industry made the discoveries which
led to the possibility of producing the necessary cellulose for the record
lacquer and the solvent for it. Is was not until 1935 that Svenska Radioaktiebolaget
considered its records good enough to put on the market.
The recording discs are made in two qualities, one more expensive with aluminium
sheet base and one cheaper with cardboard base. The lacquer is the
same in both cases. Aluminium sheet has been decided on, partly in view of
its low specific weight making the discs light to handle and partly because
of the ease in rolling aluminium sheet with the necessary evenness of surface.
Any little irregularity of the plate surface shows on the lacquer surface as
a small hollow or bulge. The grade of hardness is also accurately fixed. All
stresses must be removed so that the discs do not bulge with the further treatment,
hole-punching and cutting out. The discs must yet be stiff enough
to stand light shocks. The thickness of the discs is 0.5 mm with a tolerance
of ± 0.02 mm. After shearing and first rounding, Fig. 1, they are impregnated
before proceeding to the lacquering. The cardboard in the cheaper blanks is
glazed; after cutting out, the board material is immersed in a bath and dried
to remove air formations in the pores of the paper. Otherwise the air would
be brought to the surface by the lacquer and entirely spoil the disc.
Obviously good results in playing the record are intimately connected with
accuracy in the lacquer manufacture. To obtain a record free from scratching
it is necessary to ensure that the ingredients are of the finest quality; in
particular the cellulose must display great homogeneity and the mixture must
be kept stirring for a considerable time. The viscosity is checked and the
lacquer is coloured black to give the finished blank a more attractive
appearance. When it is dry the lacquer must not be brittle, but soft without
being doughy. The harder it is the more of the high frequencies can be taken
in but there is risk of needle scratch being too great. It is a question of hitting
on the golden mean.
After the lacquer coating of the aluminium or cardboard there remains therefore
a troublesome and prolonged drying procedure. The discs are placed in
drying chambers, Fig. 2, through which air is forced by fans with moderate
speed. The air naturally must be entirely free from dust particles and at the
air intake dust filters are inserted consisting of several layers of cottonwool
between linen cloth. The temperature and humidity are measured and corrected
if deviation is too great. It is the great relative degree of moisture in the
chamber in particular that render the drying difficult, but by appropriate air
treatment there is obtained a drying time which is not too far removed from
the normal. The time is not the same for all discs, being governed by the size
of disc. When the disc is dry the edge is cleaned and a number of holes are
punched in the centre to fit the various fixing devices employed at different
broadcast stations. After strict inspection, Fig. 3, the disc is finished.
Svenska Radioaktiebolaget make six different types of recording discs, vis:
three sizes of card discs: P 15, P 20 and P 25, and three sizes of aluminium
discs: A 20, A 25 and A 30. The figures represent the diameters in cm, and
the discs give different recording times according to the diameters. As every-
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