The Library of Work and Play: Electricity and Its Everyday Uses

by John F. Woodhull

"My garden hose will usually throw a stream across the street, which is very desirable when one wishes to sprinkle the street, but this pressure is disastrous when I wish to sprinkle the flowers. Turning down the stop-cock at the nozzle makes it shoot a smaller stream but more spiteful in pressure, knocking the flowers to pieces and washing the soil away from their roots. But if I partially close the stop-cock at the side of the house where the hose is attached I may have the stream of water flow as gently as I choose.

"I should meet precisely the same situation if I tried to ring an ordinary electric bell by a 110-volt current, and I should use the same method of overcoming the difficulty.

"The great virtue of the dynamo is that it can furnish a large supply so that the voltage is kept constant on a great flow of current.

Fig. 126

"I have not forgotten the question, but have tried to work toward its answer all this time. The question is, why did Ernest get a shock this morning when he touched only one binding post, and when the battery of five cells is not capable of giving shocks to any one who touches its binding posts directly? We need one more diagram to give the final answer. In [Fig. 126] e represents the binding post from which the shock was received. B is the battery of five cells, C is the spark coil, G is the engine cylinder, f is the spark plug. When one wishes to start the engine he closes the switch S. This makes a continuous conductor from the battery to the metal cylinder itself. As the engine rolls over it closes the gap in the conductor at d for an instant. The primary circuit is then completed and the current passes from B to the cylinder, through the metal of the cylinder to d, then to the coil C, where it passes through a portion of the coil and then back to the battery. The vibrator on the coil causes the magnetic field to rapidly vary in strength. This induces a secondary current in the whole coil which, because it passes through a very great number of turns, has a high voltage. This passes from C through B to the base of the engine, then up the walls of the cylinder to the plug f, then jumps across the gap at a, causing the spark which explodes the mixture of gasolene and air in the cylinder. The spark plug f is porcelain—an exceedingly good insulator. Through the centre of this passes a wire from a to e. The current passes up this and back to C. Now the engine rests upon the floor of the boat, and Ernest stood upon the same floor. The wood of this floor when dry and clean is a very good insulator, but when wet, and particularly when wet with water that has ever so slight an amount of any salt in solution, it becomes a conductor for such high tension currents. When therefore Ernest, standing upon the floor of the boat, touched the binding post, e, this induced current of high voltage found it about as easy to pass from the metal of the engine cylinder through the wood to his body and through his body to e as to jump across the short air gap at a. There are two things upon which he may congratulate himself.

"1. While the coil stepped up the voltage so high it reduced the available quantity of the current, so that the shock was a safe one.

"2. He received only a portion of the current which passed. The major part of it passed across the gap at a, otherwise we should have noticed that the engine missed an explosion when he touched the binding post."

The only part of this electrical outfit from which one may receive a shock is that line from e to C. The greatest difference in electric pressure is always to be found between the two extremities of the electric generator; as, for example, between the carbon end and the zinc end of the battery, the positive and negative poles of the dynamos; the right-hand and left-hand end of this coil. Since the right-hand end is connected by good conductors with the metal of the engine and with the floor of the boat and through it with our bodies, we are in the same electrical condition as the right end of the coil; but the left-hand end and the wire connecting it with e are forced by the varying magnetic field into a very different state of electric tension, and it is insulated from the engine and from us by the porcelain spark plug. We say that the "difference in potential" between the two sides of this system is 5000 to 10,000 volts.