Stránka:roll 1917.djvu/160

128 Woodcraft Manual for Boys work screwed to a hard rubber base is used to hold the crystal in place and to provide the means for obtaining the right pres— sure of the point on the crystal. TEE TUNING COIL. This is made by winding a single layer of No. 20 or 22 insulated copper wire on a cylinder of wood, glass, or other material; the insulation of the wire is scraped ofi in two parallel lines the length of the coil and two springs sliding on brass rods make contact with the turns Of wire. The tuner, as it is sometimes called, is used to tune in any sta- tion, that is to adjust the ae'rial wire and circuits of your re- ceiver to the wave length sent out by the station which you want to listen to. It is also useful to tune out interfering signals and to make the received signals ring clear and loud in the receiver. TEE CONDENSER. The best kind of a receiving condenser is of the variable type. It is made of a number of thin sheet brass TWO 5 L ID E k VAR/flBL CONDEN5ER TUNING COIL ._.q H5190 TEL E PHONE RE CE I VER Fig 4. The receiver ready to listen I]: semicircles, called leaves, half of which are fixed in position and the other half, which alternate with the fixed leaves, are made movable. The movable leaves can be turned by means of a knob and very sharp tuning can be obtained when it is used in connection with the tuning coil. Tm: TELEPHONE RECEIVERS. To receive over long distances