The Filament Rheostat.--An adjustable resistance, called a rheostat, must be used in the filament and storage battery circuit so that the current flowing through the filament can be controlled to a nicety. The rheostat consists of an insulating and a heat resisting form on which is wound a number of turns of resistance wire. A movable contact arm that slides over and presses on the turns of wire is fixed to the knob on top of the rheostat. A rheostat that has a resistance of 6 ohms and a current carrying capacity of 1.5 amperes which can be mounted on a panel board is the right kind to use. It is shown at A and B in Fig. 40 and costs $1.25.
Assembling the Parts.--Begin by placing all of the separate parts of the receiving set on a board or a base of other material and set the tuning coil on the left hand side with the adjustable switch end toward the right hand side so that you can reach it easily. Then set the variable condenser in front of it, set the vacuum tube detector at the right hand end of the tuning coil and the rheostat in front of the detector. Place the two sets of batteries back of the instruments and screw a couple of binding posts a and b to the right hand lower edge of the base for connecting in the head phones all of which is shown at A in Fig. 41.
Connecting Up the Parts.--To wire up the different parts begin by connecting the sliding contact of the primary coil of the loose coupled tuning coil (this you will remember is the outside one that is wound with fine wire) to the upper post of the lightning switch and connect one terminal of this coil with the water pipe. Now connect the free end of the secondary coil of the tuning coil (this is the inside coil that is wound with heavy wire) to one of the binding posts of the variable condenser and connect the movable contact arm of the adjustable switch of the primary of the tuning coil with the other post of the variable condenser.
Next connect the grid of the vacuum tube to one of the posts of the condenser and then connect the plate of the tube to the carbon terminal of the B or dry cell battery which is the + or positive pole and connect the zinc terminal of the - or negative pole to the binding post a, connect the post b to the other side of the variable condenser and then connect the terminals of the head phones to the binding posts a and b. Whatever you do be careful not to get the plate connections of the battery reversed.
Now connect one of the posts of the rheostat to one terminal of the filament and the other terminal of the filament to the - or negative terminal of the A or storage battery and the + or positive terminal of the A or storage battery to the other post of the rheostat. Finally connect the + or positive terminal of the A or storage battery with the wire that runs from the head phones to the variable condenser, all of which is shown in the wiring diagram at B in Fig. 41.
Adjusting the Vacuum Tube Detector Receiving Set.--A vacuum tube detector is tuned exactly in the same way as the Crystal Detector Set No. 2 described in [Chapter III], in-so-far as the tuning coil and variable condenser are concerned. The sensitivity of the vacuum tube detector receiving set and, hence, the distance over which signals and other sounds can be heard depends very largely on the sensitivity of the vacuum tube itself and this in turn depends on: (1) the right amount of heat developed by the filament, or filament brilliancy as it is called, (2) the right amount of voltage applied to the plate, and (3) the extent to which the tube is exhausted where this kind of a tube is used.