between the plate and filament circuits and so the plate circuit is completed through the ground connection.
There is one more thing that might cause you a little trouble -- the output of the rectifier tube, the tube having two plates shown in the lower right-hand corner, goes through a choke coil, then it goes through the magnet coil of the speaker before going through the plates of the other tubes. If the speaker is disconnected as shown, there can be no plate current in the receiver.
You are urged to make a careful study of the diagram in Fig. 20. The ability to trace circuits from diagrams or in a receiver is an important part of the Radio-Trician’s equipment. Become accustomed to doing it now and it will make your study of Radio a good deal easier and enable you to start servicing receivers soon.
ELECTRICAL MEASUREMENTS
In practical electricity and in Radio we have units of measurement just as in every day life we have the units, foot, yard, mile, quart, and ounce. Some of these have already been mentioned, such as the “volt” -- the unit of electrical force -- the “ampere,” the unit of current flow; and the coulomb, the quantity of electricity; the “ohm,” the unit of electrical resistance.
The pressure (voltage) and the “quantity per second” (in amperes) in a circuit can easily be determined by the use of meters (measuring devices). The “voltmeter” measures the voltages, the “ammeter” (ampere-meter) measures the current flowing. Knowing the value of the e.m.f. and the current, a Radio-Trician can make many of the calculations needed in his work--he can determine the resistance in the circuit, he can determine with the use of a voltmeter and an ammeter just where the trouble is in defective sets or what steps he must take to improve a set.
THE D.C. AMMETER
The voltmeter and the ammeter both employ principles with which you already are familiar, namely, those of magnetic attraction. Figure 22 shows the internal construction of an ammeter in sketch form. The particular meter shown is a milli-ammeter. The rectangular coil of wire is mounted over a stationary iron core, on a shaft fitted in bearings at each end so that it is free to rotate in a magnetic field produced by a permanent magnet. The armature assembly (coil on shaft) is held
