BARLOW'S WHEEL. 49 axis and set over a trough containing mercury in such way that during rotation of the wheel one or two spokes were always dipping into the mercury. Fig. 10. A permanent steel magnet N S was placed in such position that the lines of force joining its two poles passed transversely across the plane of rotation of the wheel, and upon sending a current through the wheel in the direction indicated by the arrows, rotation was produced in the opposite sense to the hands of a watch as seen from the side on which was placed the N pole of the magnet. It will be seen at a glance that this apparatus is nothing else Fig. 10. N but our arrangement of a slider in rotary form, the lines of the magnetic field being in this case horizontal where they cut through the wheel. Each spoke is a slider coming successively into action as its extremity touches the mercury in the trough and is thus put in electrical connection with the rest of the circuit. It was also found that the experiment succeeded if, instead of a star wheel, a plain metallic disc was employed, the lowest point of the circumference just touching the mercury. In 1831 Faraday reversed the experiment and obtained an electric current from a disc rotating between the poles of a magnet. Fig. 11. The magnet was so placed that the induction between the poles, that is, the lines of force E passing from one pole to the other, should pierce the surface of the disc, and the current was taken off by contact springs on the axis and on the circumference ; the latter being placed on the radius of greatest induction. Lately Professor George Forbes has constructed dynamos on the same principle, the only difference being that, instead of using a permanent steel magnet, he uses an electro-magnet which becomes excited by the current produced. Professor Forbes' machine 1 is remarkable for the very powerful current it produces as compared to its Fig. 11. 1 N small size. He has devised several modifications, but for our purpose it will be sufficient to describe one of his arrangements. The armature of this dynamo, which is illustrated in Fig. 12 in longitudinal section, consists of a wrought iron cylinder without any wire on it. The field magnet is a closed iron casing surrounding the armature on all sides, and containing two circular grooves of tapering section into which are laid the exciting coils E, formed of insulated copper wire. If a current passes through these coils, it produces lines of force which completely surround each coil, and which pass partly through the iron shell C D forming the field magnet, and partly through the armature A. The general character of 1 See "The Engineer" of July 17, 1885. The author is indebted to the editor of that paper for the use of the engraving. FORBES' NON-POLAR DYNAMO. these lines is shown by the dotted curves. 51 It will be seen that as the armature cylinder revolves it will become the seat of electro-motive forces acting at right angles to the lines, as indicated by the arrows, and if we provide rubbing contacts at the ends of the cylinder we can obtain the current due to these electro-motive forces. The contacts are arranged at the inner periphery of the exciting coils, and consist of a series of carbon blocks held in two copper rings, which are connected to the two terminal plates G G. The current is thus taken off all around the armature, and the latter contains absolutely no idle portion. This is one of the reasons why the machines are so powerful as compared to their size. The other reason is that the intensity of the magnetic field is very great. As will be shown in a subsequent chapter, when the theory of continuous current motors will be given, the intensity of the magnetic field is the greater the smaller the clearance between the polar surface of the magnet and the core of the armature. In motors or dynamos, which contain copper wire coiled over the armature core, this clearance is necessarily greater than in Professor Forbes' dynamo, where the space between armature and magnet is just sufficient to allow of free rotation. The following figures will serve to give an idea of the relation between the size of these machines and their output of electrical energy. A dynamo having an armature six inches in diameter and nine inches in length, will, when driven at a speed of 2,000 revolutions a minute, give a current of 5,000 Amperes at a difference of potential of two Volts. According to the inventor, an armature four feet in diameter by four feet in length would produce an electro-motive force of sixty Volts when driven at a speed of 1,000 revolutions a minute. If we were to allow the current to increase in the same proportion as the area of the armature cylinder, this machine could produce 320,000 Amperes, and would require about 30,000 h.-p. to drive it. The employment of such an enormous power and at the high speed of 1,000 revolutions is of course out of the question, but on purely theoretical grounds it is interesting to notice how easily our simple experiment of the slider when suitably arranged in rotary form will lead to results which on account of their magnitude are quite beyond the capability of modern engineering. Dynamos similar to that just described are generally called Uni-polar Dynamos. Professor Forbes prefers the title Non-polar Dynamos, and with good reason, for, as was pointed out already in the IDEAL ALTERNATING CURRENT DYNAMO. 53 first chapter, a magnet with only one pole is a physical impossibility. All the dynamos of this class have the disadvantage of requiring to be driven at a very high speed in comparison to the electro-motive force they can produce. The reason lies in this, that the length of conductor cutting through the field is limited by the size of the armature. These machines are practically nothing else but dynamos having only one turn of wire wound on their armature core. An ideal machine of this kind is shown in Fig. 13. The field magnets N S are placed IDEAL ALTERNATING CURRENT DYNAMO. horizontally opposite each other, and their polar surfaces are bored out to form a cylindrical cavity within which one single turn of wire can be revolved by means of a crank. One end of the wire is joined to the axis A A, and the other to a metal sleeve M, rubbing contact springs B1 B, being arranged in order to take the current off the sleeve and axis respectively. The lines of force pass horizontally across the cylindrical cavity from N to S, and those which are contained within the space swept by the wire are cut twice during each revolution. The effect is the same as if we had attached our slider to |