This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
A voltmeter, being designed to measure pressure, must be connected across the terminals of the article to be measured; for instance, to measure a dynamo, one of the terminals of the meter must be connected to one of the dynamo-terminals, and the other terminal to the other dynamo-terminal. In the same way, to measure the E.M.F. of a cell, one terminal must be connected to one plate, and the other terminal to the other plate.
The only reliable method of finding the total voltage (that is, the voltage of all the cells in series) is to connect them to the engine-room voltmeter, which is a similar apparatus to the hand-voltmeter, except that the latter is a .small portable instrument with spear, reading to about 3 volts, whereas the former is a much larger instrument, screwed to the switchboard (presently to be described), and with a scale reading to (say) 140 volts.
An ampere-meter (or ammeter) is an instrument on the same principle, except that whereas a voltmeter is bridged across the terminals of the apparatus to be measured, and has consequently to be wound with many turns of fine wire to permit only just enough current to pass through it to move the indicator, an ammeter has to be placed in the circuit, the current which it is desired to measure passing through it. If it were required to ascertain the current given out by a dynamo, it would be necessary to connect one terminal of the dynamo to the ammeter, and the other terminal of the ammeter to the circuit, or, figuratively speaking, to cut the circuit and insert the ammeter.
Both voltmeters and ammeters must be used for such circuits only as they are intended for. If the 3-volt hand-voltmeter were connected across a 200-volt dynamo, it would be burned up with the excessive current passed through it, as it was only wound with sufficient wire to withstand a pressure of 3 volts. In the same way, if an ammeter with a scale of from 5 to 20 amperes were put in a circuit carrying 1000 amperes, it too would be burned up; but if it were put in a circuit carrying .01 of an ampere, it would not indicate anything, since its scale commences at 5 amperes. The very low readings of long-range instruments are always treacnerous; soon a 150-scale voltmeter, no reliance should be put on the readings of (say) the first ton volts.