Letters of a Radio-Engineer to His Son

by John Mills

That is the way in which to compare batteries and to measure their e. m. f.’s, but you see it takes a lot of time. It is easier to use a “voltmeter” which is an instrument for measuring e. m. f.’s. Here is how one could be made.

63First there is made a current-measuring instrument which is quite sensitive, so that its pointer will show a deflection when only a very small stream of electrons is passing through the instrument. We could make one in the same way as we made the ammeter of the last letter but there are other better ways of which I’ll tell you later. Then we connect a good deal of fine wire in series with the instrument for a reason which I’ll tell you in a minute. The next and last step is to calibrate.

We know how many volts of e. m. f. are required to keep going the electron stream between n and b–we know that from the e. m. f. of our standard cell. Suppose then that we connect this new instrument, which we have just made, to the wire at n and b as in Fig. 15. Some of the electrons at n which are so anxious to get away from the negative plate of battery B can now travel as far as b through the wire of the new instrument. They do so and the pointer swings around to some new position. Opposite that we mark the number of volts which the standard battery told us there was between n and b.

If we move the end of the wire from b to d the pointer will take a new position. Opposite this we mark twice the number of volts of the standard cell. We can run it to a point e where the distance ne is one-half nb, and mark our scale with half the number of volts of the standard cell, and so on for other 64 positions along the wire. That’s the way we calibrate a sensitive current-measuring instrument (with its added wire, of course) so that it will read volts. It is now a voltmeter.

If we connect a voltmeter to the battery X as in Fig. 16 the pointer will tell us the number of volts in the e. m. f. of X, for the pointer will take the same position as it did when the voltmeter was connected between n and d.

There is only one thing to watch out for in all this. We must be careful that the voltmeter is so made that it won’t offer too easy a path for electrons to follow. We only want to find how hard a battery can pull an electron, for that is what we mean by e. m. f. Of course, we must let a small stream of electrons flow through the voltmeter so as to make the pointer move. That is why voltmeters of this kind are made out of a long piece of fine wire or else have a coil of fine wire in series with the current-measuring part. The fine wire makes a long and narrow path for the electrons and so there can be only a small stream. Usually we describe this condition by saying that a voltmeter has a high resistance.

Fine wires offer more resistance to electron streams than do heavy wires of the same length. If a wire is the same diameter all along, the longer the length of it which we use the greater is the resistance which is offered to an electron stream.

65You will need to know how to describe the resistance of a wire or of any part of an electric circuit. To do so you tell how many “ohms” of resistance it has. The ohm is the unit in which we measure the resistance of a circuit to an electron stream.