CHAPTER LXI. MISCELLANEOUS APPLICATIONS OF ELECTRICITY. 851. Electro-magnetic Engines.-Electro-magnetic engines, more briefly styled eléctromotors, are machines driven by currents; their action depending on mechanical forces called out either between magnets and magnets, or between magnets and currents, or between currents and currents. Since their first construction in 1834 by Jacobi of St. Petersburg, who propelled a boat on the Neva by means of one, they have remained in the position of scientific toys till the recent electrical revival; but they are now becoming important in connection with the electrical transmission of driving power to a distance. A waterfall or a fixed steam-engine can be employed to generate a current of electricity by means of a dynamo or other magneto-electric machine, and this current can be made to propel a carriage or drive machinery at a distance by means of an electromotor, which receives the current and reconverts it into mechanical effect. There is always some waste of energy in this double process of conversion; but the waste is less than by any other mode of transmitting power, if the distance be considerable. The attempts of early inventors to bring such machines, with galvanic batteries supplying their currents, into competition with steam-engines, necessarily resulted in failure, on the score of expense. A current can be more cheaply produced by a steam-engine driving a dynamo machine than by a galvanic battery; and less work would be obtainable from an electromotor driven by this dynamo than from the steam-engine direct. 852. The most successful electromotors hitherto employed have been continuous-current dynamo machines, such as that of Siemens (Fig. 550), or of Gramme (Fig. 555), used not as dynamo machines but in the converse manner. To explain how the motion is produced, we may begin with a somewhat simpler case-that of the original Siemens' armature depicted in Figs. 545, 546. If a current from without be sent through the coil of this armature so as to make it an electro-magnet with a and b (Fig. 546) as poles, there will be a dead point in the position represented in the figure, where the fixed pole A of the field magnet is repelling a and the other ELECTRO-MAGNETIC ENGINES. 833 fixed pole B is repelling b; but if the armature be slightly displaced from this position these two repulsions will concur in rotating the armature through about 180°. By the action of the commutator (Fig. 547) the current is interrupted for an instant at each of the two dead points (which are 180° asunder), and is then again supplied in such a direction that the pole which has just passed a pole of the field magnet is repelled by it, the repulsion lasting till the next interruption of the current. This armature is actually employed in the manner here described for some kinds of light work such as the driving of sewing-machines; but the occurrence of the dead points is a disadvantage. This difficulty could be surmounted by employing two such armatures in positions differing by 90° from each other; but still greater steadiness is obtained by employing the arrangement of Fig. 550, which may be regarded as a combination of a number of such armatures with their poles ranged at equal distances round the circumference of the circle described. Again in the case of the Gramme ring (Fig. 552) it is clear that if a current were sent into the coil at C from an external source and drawn off at E, this current would divide itself into two parts, one going through CDE and the other through CFE. If we suppose C to be the top and E the bottom, the current both at D and at F will circulate in the direction of watch-hands; hence both halves of the coil combine to give one pole at C and the other at E. The attractions and repulsions between these poles and the poles P P' of the field magnet will constantly tend to turn the ring in one direction; for instance, if C is similar to P, C will be urged to the right and E to the left. The rotations in all these cases might also have been deduced from the tendency of wires conveying a current to move across tubes of magnetic force. The two explanations in fact are fundamentally identical. 853. Quantitative Relations. The following investigation shows the relations which exist between the work employed in driving the generator and the work given out by the motor. Let E denote the e.m.f. of the generator (that is of the dynamo employed to generate the electricity), and e the reverse e.m.f. of the motor (which is itself a dynamo worked backwards). The whole e.m.f. in the circuit is E-e, and if R denote the whole resistance of the circuit, the expression for the current will be In each unit of time the quantity CE of mechanical energy is converted into electrical energy in the generator, and the quantity Ce of electrical is converted into mechanical energy in the motor. The efficiency, as measured by the ratio of the mechanical energy given out to that put in, is accordingly, and the work wasted is C(E-e), which, on reference to the value above obtained for C, will be seen to be equal to C2 R, the well known expression for the heat generated. The expression for the efficiency shows that for economical working, the reverse e.m.f. should be a large fraction of the direct. On the other hand, to obtain the greatest amount of work from the motor in a given time, when E and R are given, the product e (E-e) must be made a maximum, that is, E must be divided into two parts whose product is the greatest possible; hence the two parts must be equal, e will be E, and the efficiency will be . 854. Economy in Transmission.—-When large quantities of electrical energy are to be transmitted to great distances, whether for driving motors or for supplying electric lamps, it is important that the work spent in heating the intervening conductor should be as small as possible. The expression for this work is C2r, where r denotes the resistance of the conductor. This latter factor can be diminished by increasing the size of the conductor, but a stout rod of copper is very expensive when the distance is several miles. The other factor C2 can be diminished without change in the whole amount of energy, if we at the same time increase the electro-motive force E, so as to keep the product CE unchanged in value. If electricity is ever to be transmitted with commercial success over such distances as 50 or 100 miles, it must be by the employment of excessively high electro-motive forces. The objections to high electro-motive force are the increased tendency to leakage, and the more dangerous character of the shock which will be received by a person inadvertently touching the conductor. From 150 to 200 volts (see page 852) is the limit which the Board of Trade has hitherto imposed on circuits liable to be touched by the public, whereas 2000 to 2500 volts have been advantageously employed in electric light installations. To obtain economy combined with safety, means have been devised for transforming currents of high tension into larger currents of lower tension. This has been done in the three following ways. TRANSFORMATIONS OF CURRENTS. 833* 854A. Transformation of Currents.-The first method consists in employing the original current to charge storage cells arranged in a long series, and afterwards rearranging them in shorter series. The second consists in employing the original current to supply an electromotor which drives by mechanical means a low-tension dynamo. The simplest mode of making the connection is to mount the armature of the motor and the armature of the dynamo on the same axle. These two methods are applicable to direct currents. In the third method, which is specially applicable to alternate currents, an apparatus is employed consisting of a primary and a secondary coil wound together over an iron core, the secondary consisting of much stouter wire than the primary, and having about the same total weight. The original alternating current is passed through the primary, and generates by induction a larger current of lower tension in the secondary. This apparatus is technically called a transformer, and much attention has been given to the improvement of it in recent years. The primary and secondary conductors should be closely interwound, and the iron core should not have free ends, but should form a closed circuit (for magnetic lines of force) embracing the coil. In some of the most efficient transformers it is so extended as almost completely to surround the coil. It is now a common thing for electricity, like gas, to be supplied to a number of houses through "mains" proceeding from a central station; the “mains" consisting, in this case, of stout copper wires well insulated and laid in pairs, one wire for the direct and the other for the return current. In connection with such systems of electrical supply, it is usual to employ one of the above modes of transformation, the transformer being interposed between the mains and the houses. 855. Froment's Engine.-Of earlier forms of electromotor the best known perhaps is Froment's. It may be described as consisting of a wheel, with eight armatures of soft iron attached to its circumference at intervals of 45°, rotating under the action of four electromagnets fixed to a cast-iron frame at intervals of 60°. Each magnet is "made" when an armature comes within 15° or 20° of it, and "unmade" as the armature is passing it. The making and breaking of the circuits is effected by means of three distributors, one of which is shown on an enlarged scale in Fig. 581. R is an eight-toothed wheel, fixed to the axis on which the armatures revolve, and turning with them. Each tooth, as it passes the roller r, pushes it away, and brings the studs m'm into contact. As long as they remain in contact, the current circulates through the coil with which the distributor is connected. The distributors are screwed R. m m Fig. 581.-Distributor. into a metallic arc, which is constantly connected with one pole of the battery. One of them serves for the two opposite horizontal magnets, which are made and unmade together. The two lower magnets have one distributor apiece. Matters are so arranged that the current is not cut off from one coil till just after it has commenced to flow in the next. This precaution prevents, or at least mitigates the induction-spark which generally occurs in breaking circuit (§ 814), and which has the mischievous effect of oxidizing the contacts, and thus, after a time, deranging the movements. 856. Edison's Electric Pen.-A good example of the production of mechanical power on a small scale by means of electricity is furnished by Edison's electric pen. A miniature battery sends a current, through a flexible cord consisting of two insulated wires twisted together (one |