In [Fig. 123], suppose A represents a small, slim tank of water three feet high. The water-wheel W, requires one gallon of water a minute pushed along by a three-foot head of water pressure to run it. The supply pipe S is bringing into the tank not more than one quart of water per minute. A gate at R enables us to regulate the flow of water, as we regulate the flow of electricity, by using more or less resistance. Now it is evident that if we close the gate, or partially close it, and allow the tank to fill with water, we may then open the gate and run the wheel for a short time, but the level of the water in the tank soon begins to fall and the pressure grows less and the wheel stops moving. It is just so with all generators of electric current. If we take from them more than they can supply continuously the voltage falls. This is notoriously true of dry cells. Like the water tank represented in [Fig. 123], they "run down" if used continuously to furnish, say, one ampere of current, but they may furnish it for a short time, the voltage rapidly falling meanwhile. Then if given a short rest they "pick up" and will again furnish full pressure. The voltage of a dry cell falls somewhat when it is required to give the very small amount of current required to actuate a volt meter, say .015 ampere. Hence, our volt meter will not quite correctly show what the voltage of a single cell would be on open circuit. Notice that, when I put one cell upon this volt meter the needle shows 1.42 volts; but when I put four cells in series upon it the needle indicates six volts, as nearly as we can read it. That is, the voltage of each cell in this case appears to be 1.5. What has increased the voltage of a cell from 1.42 to 1.50? Simply this: when .015 ampere, the amount required by the volt meter, was taken from one cell it reduced its pressure, but when a multiplier with ten times the resistance was added we secured our reading by using only .006 ampere of current, and this did not appreciably reduce the true pressure of the cells.
The induced current from our bell when held back by 60,000 ohms of resistance in the four boys was able to push with 150 volts of pressure, and .0025 ampere passed without noticeably reducing this pressure, but when the same current was held back by only 100 ohms in the filament of the lamp nearly forty times as much current passed, and the pressure dropped to something less than ten volts.
"We will try an experiment to show how the voltage will suddenly fall when we reduce the resistance of your four bodies.
Fig. 124
"Fill these two empty tin pails in which our lunch was brought with water from the lake and sprinkle in the salt left over from the lunch. Now twist a bare copper wire around the bail of each pail and connect these with the bell so as to get the induced current from its magnet. (See [Fig. 124].) Let the two pails of water be the terminals of the two wires at S. Now you four boys wet your hands in the water and then join hands, and those at the two ends of the line put your free hands upon the outside of the pails of water while I close the primary circuit. You of course feel the current just as you did when you held the spikes in your hands in a former experiment. But now you two end boys put your free hands into the salt water, and you instantly get a very smart shock. The resistance is no longer 60,000. It has dropped way down to 2000, and if the voltage had remained at 150 you would have received a terrible shock, but the voltage has dropped down to five. It is as though you had been pushing very hard against a post and it suddenly gave way. You cannot push against a thing which offers no resistance. So the voltage falls when resistance is reduced, and particularly if the source of supply has very little capacity. Here is another experiment you must try when you go back to the city. At a certain water faucet in my laboratory the pressure is disagreeably high. The water flows with great force and spatters badly. We can easily reduce the pressure so that the water will flow in a limpid stream. [Fig. 125] shows the situation; f is the faucet, and in the pipe underneath the sink there is a stop-cock c. This may be adjusted permanently so that the faucet f will act pleasantly. The same thing is represented again at the gas stove. Let f in the [Fig. 125] represent a gas cock at the stove. Suppose the pressure is so high that the gas flames pass more gas than is readily consumed. It is possible to adjust a stop-cock like c further back in the pipe so as to produce hotter flames, get rid of the poisonous fumes of half burned gas, and cut down the monthly gas bills one half.
Fig. 125