April 2 1, 1881] 



NATURE 



58r 



Faraday also observed the difference of time between 

 induction by a battery current in a coll, which is instan- 

 taneous, and induction by a magnet, which requires an 

 interval of time to get up to its full value ; and accounted 

 for this retardation by supposing that there is a redistri- 

 bution of the Amperian currents in the iron itself, so that 

 the magnet requires time to rise to its full power. 



Little did Faraday dream of the rapid development and 

 the marvellous results which were to flow fro.n his experi- 

 ments when, fifty years ago, he established the laws of 

 magnetic and current induction, being stimuUted (as he 

 says) to investigate experimentally the inductive effects 

 of electric currents with the view of elucidating Ampere's 

 beautiful theory of magnetism, and in the hope of obtain- 

 ing electricity from ordinary magnetism. 



Lecture II. — It was shown in the list lecture that a circu- 

 lar current or a current in a coil of wire acted as a magnet, 

 one face of the coil, in which, as we look at it, the current 

 appears to go contrary to the hands of a watch, corre- 

 sponding to the marked pole or the pole of a magnet 

 which points to the north, and the opposite face of the 

 coil corresponding to that pole of the magnet which 

 points to the south. Each of the Amperian circular 

 currents in the separate molecules of a magnet is equiva- 

 lent to a fine magnet with poles of the same magnetic 

 strength as the current, and occupying the same position, 

 and the collection of Amperian currents will have the 

 same magnetic effect as the bundle of small magnets, 

 each of which gives the direction of the magnetic force 

 at the point. These separate fine magnets may be re- 

 garded as Faraday's lines of force, and the number of 

 them issuing from a magnetic pole will be a measure of 

 the strength of the magnet. The magnetic field of the 

 magnet is any portion of space to which the influence of 

 the magnet extends. The current which will be produced 

 by the motion of a conducting wire in the magnetic field 

 will be proportional to the strength of the magnetic field, 

 i.e. proportional to the number of lines of force cut by 

 the conductor ; so that the current produced in each half 

 turn of a coil of wire revolving on an axi? is projiortional 

 to the number of lines of force cut by the coil during its 

 rotation, so that the total current from the coil will be 

 proportional to the number of lines of f >rce multiplied 

 by the number of times the wire is repeated in the coil. 

 In the case of a coil of wire rotating in the field of a 

 magnet, if the axis of rotation is parallel to the lines of 

 force no current is produced, but as the axis of rotation 

 is turned more and more nearly at right angles to the 

 lines of force the current in the coil is increased. Taking 

 the earth for our magnet, when the axis of rotation is 

 perpendicular to the lines of force and still in the mag- 

 netic meridian, the current in that half of the coil which 

 is moving from west to east will be from north to south, 

 and the current in the other half of the coil which is 

 moving from east to west will be from south to north, so 

 that in the whole coil we get during every half turn an 

 all-round current in one direction in the coil. The direc- 

 tion of the current in the coil, as we look at it from the 

 east, is the same as the direction of rotation of the coil 

 as we look at it from the north. The direction of the 

 current in the coil is alternately in opposite directions for 

 every half turn, but a continuous current may be obtained 

 from it by reversing the connections with the ends of the 

 coil by a commutator at the same instant as the currents 

 are reversed in the coil. 



These are the principles of all magneto-electric ma- 

 chines. The distribuiion of lines of force in the magnetic 

 field of currents and of magnets is well shown by pro- 

 jecting some of Prof .S. P. Thompson's transparencies, 

 which show the magnetic effects resulting from the mutual 

 action of currents and magnets on one another. 



After the discovery by Faraday, in 1831, of the method 

 of producing a current of electricity by the sudden 

 removal of a coil of wire from the pole of a magnet, the 



laws of these currents were being developed, but for 

 twent\- years no attempt was made to appl)- them for the 

 purposes of electric lighting. Voltaic batteries were 

 being improved, and the more constant batteries of 

 Uaniell, of Grove, and of Bunsen were discovered, and 

 these were the sources of electricity employed to produce 

 the more powerful currents of electricity. In this country 

 forty or fifty cells of Grove have given us the electric 

 light for optical experiments in our laboratories, and the 

 light was kept steadily in the same position by the 

 elaborate arrangements of wheel-work and electromagnets 

 devised by Staile in 1847 and by Foucault, which 

 have reached very great perfection in the hands of 

 Duboscq. In the Duboscq lamp the current passes 

 always in the same direction, and the positive carbon 

 becomes hollowed out, and burns away about twice as 

 fast as the negative carbon, which becomes pointed. The 

 carbons are moved towards one another by means of a 

 drum carrying two wheels, whose diameters are as 2 to i, 

 which move two racks which bring the carbons together 

 This lamp is especially adapted for projection on a screen, 

 and we may study the forms of the carbons by projecting 

 them, and also study the kind of light given out by the 

 vapours of metals burning in the arc ; if we burn silver in 

 the arc we shall sec that it is rich in the violet or chemical 

 rays, which points to the reason why the salts of silver 

 are so much used in photography. 



Even with constant batteries there is gi-eat variation in 

 the steadiness of the electric light, but much more is this 

 the case when the current of electricity is obtained by 

 the motion of a coil of wire in a magnetic field, for every 

 alteration in the resistance in the circuit reacts on the 

 machine producing the current so as to increase the dis- 

 turbance. Hence regulators are necessary in order to 

 control the current, so as to keep the light constant. 

 In electric lighting regulators may act on the electric 

 lamps themselves so as to give a steady current between 

 the carbons by keeping them the same distance apart, or 

 regulators may be used in another part of the circuit to 

 control the current automatically by causing it to intro- 

 duce extra resistance when the current increases, and to 

 diminish the resistance when the current diminishes. 



Various methods of regulating the current, including 

 those employed by Siemens, Lane Fox, and Edison, were 

 then described. In order to find the yield or eftective 

 work of batteries or magneto-electric machines and their 

 efficiency, measurements of the current and of the work 

 done by the current must be made. 



There are four principal methods of measuring electric 

 currents : — 



1. The galvanometer method, by which with a galvano- 

 meter of small resistance a very small fraction of the 

 current is measured, and any error of observation is 

 multiplied in estimating the total current flowing. 



2. The heat method, in which the current is measured 

 by the heat developed by the current in a given resist- 

 ance in the circuit according to Joule's law, that the heat 

 is proportional to the square of the strength of current. 



3. The electrometer or potentiometer method, in which 

 the difference of potential between two points in the 

 circuit with a given resistance between them is directly 

 measured and the current deduced from (dim's law. 



In using Thomson's quadrant electrometer for strong 

 currents, the needle and one pair of quadrants should be 

 connected together, so that the deviation is then propor- 

 tional to the square of the difference of potential, and 

 this method is applica'ile for continuous currents and also 

 for alternate currents. By means of two electrometers 

 in different pans of the circuit the current and also the 

 work done by it may be at once measured. If, for instance, 

 one of the electrometers be connected with the two 

 carbons, the difference of potential and work done 

 between the carbons may be determined. By such 

 measurements it has been found that there is an electro- 



