Page:EB1911 - Volume 21.djvu/490

Rh the wax cylinder is rotating, the point of the marker is angled downwards, and this cuts deeply into the wax; and when there is diminution of pressure the point is angled upwards, so as to act less deeply. In reproducing the sound, the blunt end of the marker runs over all the elevations and depressions in the bottom of the groove cut on the wax cylinder. There is thus increased pressure transmitted upwards to the glass disk when the point runs over an elevation, and less pressure when the point runs over a depression on the wax cylinder. The glass disk is thus, as it were, pulled inwards and thrust outwards with each vibration, but these pulls

a.—Exterior of Edison Phonograph.

and thrusts follow each other so rapidly that the ear takes no cognizance of the difference of phase of the vibrations of the glass plate in imprinting and in reproducing. The variations of pressure are communicated to the glass plate, and these, by the medium of the air, are transmitted to the drum-head of the ear, and the sound is reproduced with remarkable fidelity. It is necessary for accurate reproduction that the point of the marker be in the centre of the groove. In the older phonographs this required accurate adjustment by a fine screw, but in newer forms a certain amount of lateral oscillation is allowed to the marker, by which it slips automatically into the groove. Two other improvements have been effected in the construction of the instrument. A powerful triple-spring motor has been substituted for the electric motor, and the circumference of the wax cylinder has been increased from 6 in. to 15 in, whilst the disk is 12 in. in diameter. The cylinders make about two revolutions per second, so that with the smaller cylinder the point of the marker runs over nearly 14 in in one second, while with the larger it runs over about 30 in. The marks correspond in to the individual vibrations of tones of high pitch are therefore less likely to be crowded together with the larger cylinder, and these higher tones in particular are more accurately reroduced in a form of instrument called the 200-thread machine motion of the drum bearing the cylinder was taken off a screw the thread of which was 50 to the inch, and by a system of gearing the grooves on the cylinder were 200 to the inch, or of an inch apart. ft [sic] was somewhat difficult to keep the marker in the grooves when they were so close together; and the movement is now taken directly off a screw the thread of which is 100 to the inch, so that the grooves on the cylinder are of an inch apart. Thus with the large cylinder a spiral groove of over 300 yds. may be described by the recorder, and with a speed of about two revolutions per second this distance is covered by the marker in about six minutes. By diminishing the speed of revolution, which can be easily done, the time may be considerably lengthened.

In the plate machine the disk is fixed to a table which is rotated at a fixed speed of about 76 revolutions a minute. The speed of the lateral movement of the table is also uniform, and by a regular progression brings the wax blank under the sound box to the sapphire cutting point, which detaches a fine unbroken thread of wax as it cuts into the surface of the blank to a depth of 3 to 4-thousandths of an inch beginning at about half an inch from the circumference and continuing the spiral groove to within a couple of inches of the centre, according to the length of the music to be recorded. The essential difference between the disk and cylinder machine is that in the former the waves are recorded by horizontal motion over the disk, while in the latter the waves are recorded as indentations.

The following is the modus operandi of making a record. The person making the record sings or plays in front of a horn or funnel used for the purpose of focusing the sound-waves upon the diaphragm. The artist and the funnel are on one side of a screen and the recording apparatus in charge of an operator on the other. The arrangement of the various instruments in the recording room at proper relative distances from the horn is of the utmost importance in order to preserve the balance of tone. At about 4 ft. from the horn are grouped the violins and the wood wind (flutes, oboes and clarinets); behind the brass wind (horns, trumpets, trombones and tubers), and right at the back the violoncellos and double basses and the kettle-drums and other instruments of percussion which may be required. On the other side of the screen is the sound-box and the recording cylinder or disk.

Cylinder records are duplicated by taking a plaster cast, electroplating, and then using it as a matrix. The disk record admits of similar treatment. After dusting with graphite it is electroplated to about 9 mm. thick. This forms the permanent or master record, from which the working negatives are made by taking wax impresses of it and obtaining copper electros in turn from them. The matrix is then nickel-plated and polished and is ready for use in pressing out the commercial records by means of an hydraulic press, the material used being a tough and elastic substance containing shellac and other compounds such as wood charcoal, barium sulphate, earthy colouring matters and cotton flock.

There is still a defect to be overcome in the gramophone, and that is the hissing of the needle produced by friction both during recording and intensified in reproduction. In one device for remedying this the stylus acts like a stylographic pen, depositing on a polished surface a fine stream of some liquid which solidifies and hardens very rapidly, forming a sinuous ridge instead of a groove in a wax blank. A negative is taken of the record and the matrix is made from it in the usual way.

b.—Mechanism of Edison Phonograph.

The auxeto-gramophone or auxetophone, patented by Short in 1898 and improved by the Hon. C. A. Parsons, is similar in scope to the gramophone but attains its results in a different manner. In the Parsons-Short sound-box there is no diaphragm, but a