pulley for strap-belting, by means of which the water or steam power used is made to turn the dynamo machine. But we must not forget the motor as a companion of the dynamo, as its indispensable brother, in turning to practical account the electricity sent to it. As we have seen, the motor is the reverse of the dynamo, at least in its effects. It is fed by the dynamo, and it imparts its power to the machinery which it is to set in motion. It is to the dynamo what the water-wheel is to the water. In one sense it is an even older invention than the dynamo, but its extended commercial application was not possible until the dynamo had reached certain stages of perfection. It is generally agreed that the first motor of importance was that constructed by Professor Jacobi, through the liberality of the Czar Nicholas, of Russia. Jacobi used two sets of electro-magnets, by means of whose mutual attraction and repulsion he rotated a wheel on a boat with a power equal to that of eight oarsmen. But as Jacobi’s electro-magnets required an electric current to magnetize them, and as there were then no means of producing such current except by the costly use of the voltaic battery, his invention was unripe as to time. In 1850, Professor Page, of the Smithsonian Institution, constructed a motor which worked ingeniously, but was still open to the objection of cost in supplying the necessary electric current for the electro-magnets. Though various inventions came about having for their object a commercially successful motor, such a thing was impossible till Gramme produced his improved and effective dynamo in 1871. This dynamo was found to work equally well as a motor, and hence it became necessary for electricians to greatly enlarge their understanding of the nature of electro-magnetic induction. They soon discovered many curious things respecting the behavior of induced currents, with the result that rapid and simultaneous improvements were made in both dynamos and motors. One of the most curious of these discoveries was that a motor automatically regulates the amount of current that passes through its circuit in proportion to the work it is called upon to do; that is, if the work the machine has to do is decreased, the motor attains a higher speed, which higher speed induces a counter electro-motive force sufficient to check up the amount of current passing through the motor. So when the motor is required to do increased work, the machine slows up; but with this slowing up, the counter electro-motive force decreases, and consequently the current passing through the motor increases.