If we want to carry only a small current for a long distance we do not need to use large cells, but many of them. We increase the pressure or voltage by increasing the number of cells set up in series. If we have a wire of given length and resistance and find we need 100 volts to force the right amount or strength of current through it, and the electromotive force of the cells we are using is one volt each, it will require 100 cells. If we have a battery that has an E. M. F. of two volts to the cell, as the storage-battery has, fifty cells would answer. If we want a very strong current of great volume, so to speak, for electric light or power, and use a galvanic battery, we should have to have cells of large surface and lower resistance both inside and outside the cells. When dynamos are used they are so constructed that a given number of revolutions per minute will give the right voltage. In fact, the dynamo has to be built for the amount of current that must be delivered through a given resistance. The same holds good for a dynamo as for a galvanic battery. If any one factor is fixed, we must adapt the others to that one in order to get the result we want. There are many other units, but to introduce them here would only confuse the reader. The advanced student is referred to the text-books. With this much as a preliminary we are prepared to take up the applications of electricity, which to most people will be more interesting than what has gone before. CHAPTER X. THE ELECTRIC TELEGRAPH. In the year 1617 Strada, an Italian Jesuit, proposed to telegraph news without wires by means of two sympathetic needles made of loadstone so balanced that when one was turned the other would turn with it. Each needle was to have a dial with the letters on it. This would have been very nice if it had only worked, but it was not based on any known law of nature.