QST/April 1916/Amplifying Receivers with Mica Diaphragms
0VER a year ago the writer introduced into the amateur field a new type of Radio receiver many times more sensitive than those of the ordinary type. The theory or principle of operation of the new receiver appears to be generally unknown among the amateurs. Reference to Sketch 1 will assist in a better understanding of its operation. If we arrange a permanent magnet with double pole pieces NN and SS opposite the ends of an armature AB, rocking on pivot C, and wind a coil on he armature a slight current through this coil will magnetize the armature and one of its ends will become North and the other South polarity. The North pole of the armature will be attracted by the South and repelled by the North pole of the permanent magnet, and the South pole of the armature attracted by the Northland repelled by the South pole of the permanent magnet, Thus we have four magnets acting on the polarized armature. Lever E serves to connect the armature with the diaphragm D and a small wire spring on the end B of the armature balances the tension of the diaphragm.
In the actual construction the coil is not wound on the armature but merely surrounds it with a very small clearance. The armature and lever are very small and lightly constructed and the diaphragm is made of Mica to reduce its weight and period. The mica diaphragm and lever arrangement is somewhat similar to the reproducer used on a graphophone.
The resistance of the winding surrounding the armature is generally 1000 ohms and the resistance of the complete set 2000 ohms, there being only one coil in each receiver. The cord connections are inside of the receiver case in the center of the circular permanent magnet.
The exterior appearance of the Amplifying receivers is much the same as that of the ordinary type. The headband however, is of entirely new construction and has been found to be very easily adjusted, very comfortable, fits any head regardless of size, and does not pull the hair. There are no screws or nuts to set and simplicity rules throughout. See photograph cut No. 2.
Tests made in the Research Laboratory of one of the large commercial companies resulted in finding that the Amplifying receivers required only one-ninth the energy necessary with the best grades of the ordinary wireless receivers to produce an audible signal. This means that the Amplifying Receivers are nine times as sensitive as those of the regular construction.
The type of detector used has nothing whatever to do with the sensitive qualities of these receivers. Whether used with an Audion, crystal or any other form of detector the results will always be far better than with ordinary receivers. Signals that cannot be heard with ordinary receivers can be read with the Amplifying receivers. If a pair of ordinary 2000 ohm receivers are connected in series with a pair of 2000 ohm Amplifying receivers and a faint station tuned in while listening on the Amplifying receivers and then the regular receivers placed over the ears nothing will be heard. If, however, a faint station is heard in the ordinary receivers and then the Amplifying receivers used strong signals will be heard.
In conclusion the writer will quote from one of many letters received almost daily. Mr. A. L. Groves, Brook, Va., is getting some excellent results, He writes as follows:—
“I have been getting such fine results with your Amplifying receiver for the past few months that I cannot help telling you about it. Using your Amplifying receiver, crystaloi detector and all other instruments of my own make I can hear several of the high power stations n Europe and on the Pacific Coast and the Marconi Station KIE in the Hawaiian Islands, which gives me a range of over 5,000 mile.; On the morning of December 19th, 1915 I heard two low power stations in Alaska on the, 600 meter tune a distance of about 3,800 miles. This was of course freak work but from the strength of their signals I am pretty sure that if I had been using anything but your Amplifying receiver and the Crystaloi detector I would not have heard them. Many people seem to think that I cannot hear 5,000 miles with a crystal detector, but it is true never-the-less, for I very often hear stations in Europe and have no trouble at all hearing KIE, Koko Head Hawaii, in the early morning. Sometimes their signals die out shortly after sunrise here but can hear them again after the sun gets about 1½ hours high. The same may be said of KET, Bolonis, Cal., but in addition I can hear their signals in the early evening, a long time before sun-set over there.”