JUNE 30, 1923 THE WIRELESS WORLD AND RADIO REVIEW 409

A NEW LOUD -SPEAKING DEVICE

A NEW loud -speaking device of novel design and having some features of special interest, has lately been devised

by Mr. S. G. Brown, and whilst the instru- ment has not, up to the present, gone beyond the stage of experimental models, it has never- theless been recently demonstrated at the Royal Institution, and, through the courtesy of Mr. S. G. Brown, the author has had an opportunity of inspecting the instrument and hearing its performance.

The apparatus serves not only as a loud speaking reproducer, but at the same time also acts as an amplifier and provides for a very considerable degree of amplification.

The operation is entirely mechanical in principle, and depends for its functioning on friction between a revolving glass disc and a shoe, the under side of which carries a thin layer of cork which makes rubbing contact with the glass disc. The shoe is connected to the reed of a Brown telephone ear -piece of the usual pattern, and the movement of the reed, controlled by incoming signals, serves to vary the pressure of the shoe on the revolving glass disc, thereby varying the fric- tion between the shoe and the disc.

The shoe is connected mechanically to the centre of the diaphragm of a sound box of gramophone or similar pattern.

The arrangement of the instrument recalls somewhat the Johnsen -Rahbek loud speaker, except that in the latter case the friction between the shoe and the rotating disc or cylinder is controlled electrostatically, whereas in the new instrument this friction is controlled entirely by mechanical means.

The accompanying illustrations will give some idea of the arrangement of the parts.

The rotating glass disc " G " must of course be very carefully ground and polished to ensure the utmost evenness of surface.

The speed at which this disc is rotated is somewhere in the neighbourhood of 70 revolutions per minute, so that the usual gramophone motor can be suitably employed for the purpose. The diameter of the disc in the model which has been exhibited is about 7 ins.

The shoe " S " is a steel disc about half an inch in diameter, which is rigidly attached by means of a steel wire Cc W" to the reed of the

diaphragm. This wire is about I in. in length. On the under side of the shoe which makes

contact with the glass disc is secured a thin disc of cork such as is used for tipping cigar- ettes. The telephone earpiece must, of course, be very accurately adjusted above the re- volving disc, and the shoe should be arranged so that it barely touches the revolving disc ; any movement of the reed will bring it into closer contact and produce a rubbing friction.

The " Frenophone " Mechanical Amplifier and Loud Speaker.

A spiral spring is provided to maintain the tension of the cords on which the metal shoe is slung and connected to the sound - box diaphragm.

It is essential that the glass disc should be kept free from dust or any tendency to sticki- ness, and it is therefore advisable to provide a cover to enclose that part of the apparatus.

As to the performance of the instrument, the quality of reproduction is surprisingly good, and it would not be an exaggeration to say that it threatens to rival any loud -speaking device at present on the market.

As to its efficiency as an amplifier, the degree of amplification might be estimated as ap- proximately equal to two stages of low frequency amplification. Speech from 2 LO which, with a receiver having a detector valve and one stage of low frequency amplification, giving signals in the telephones which were just audible at 2 feet from the telephones, when passed into the loud speaker gave a volume of sound with an almost entire absence of distortion, sufficient to fill a large room.

One might have expected that the varying pressure of the shoe on the rotating disc of glass would have produced a damping effect on the telephone reed as would have produced distortion, but in practice there appears to be no suspicion of any such trouble.

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