Fig. 144. Parts of a Holtzer Cabot Receiver.
In carrying out any of these suggestions remember to work with one receiver at a time, keeping the other for comparison, so that by repeated tests you may tell whether or not an improvement is being made and when well enough is reached, let it alone.
Fig. 144 shows the construction of a telephone receiver manufactured by the Holtzer Cabot Co. of Brookline, Mass. The permanent magnets and bobbins are mounted in a metal cup, B, which supports the diaphragm, A. The metal cup is enclosed in a hard rubber shell, C, and fitted with a cap, D.
The complete receivers are mounted on an adjustable headband and fitted with pneumatic ear cushions which make them set more comfortably and shut out extraneous noise. These receivers are wound to all resistances used in the wireless field but, for the experimenter, those having a resistance of 1000 ohms apiece will give the best all around results.
Fig. 145. Holtzer Cabot Head Set.
It is very desirable that the receivers should be fitted with a comfortable headband which will hold the telephones in close adjustment to the ears. Fig. 146 shows the construction of such a band.
Fig. 146. Adjustable Head Band.
Two brass straps, 1 inch wide, 12 inches long and 1/16 inch thick are necessary. Two 1/8-inch holes, mm, are bored in them 3/16 inch from one end and the end bent up at an angle along a line 3/8 inch from the end as shown by the drawing. A cover is made for the straps, by sewing two strips of leather 1 1/4 inches wide and 12 inches long, together along their edges. The covers may then be slipped over the straps. Two 1/16-inch brass strips, 3/8 inch wide and a little longer than one-half the circumference of the watch-case receiver to be fitted to the head band, are bent into stirrups as shown in the illustration by Y. The ends of the stirrups are fitted with two pivots, PP, which are riveted in a hole. Two holes or recesses are bored in the shell of each receiver, on the sides 180 degrees apart. The pivots, PP, fit into these holes and form bearings in which the receivers can turn and adjust themselves to the ears. The stirrup has two small holes, hh, bored 3/8 inch apart at the top and tapped to receive a small screw. The screws pass through the holes, mm, and hold the stirrups at the end of their respective strap.