FLF.CTROMOTIVE FORCE.] ELECTRICITY 103 to E, the intensity of which, under given atmospheric conditions and given state of the macliine, will vary as the angular velocity, being independent, within very wide limits, of the resistance 1 between D and E. It is not difficult to give a general account of the action of this machine, although it is very hard to assign the precise importance of the individual parts, very slight modifications of which greatly affect the efficiency. Suppose JD and E in contact ; the right- hand armature, charged +, acts by induction on the right-hand comb, causing - electricity to issue from the points upon the disc. At the same time the positive electricity of the right comb passes through DE to the left comb, and issues from its teeth upon the parts of the disc at the other end of the horizontal diameter. This + electricity electrifies the left armature by induction, + electri city issuing from the blunt point upon the further side of the rotating disc. The charges thus deposited on the disc are carried along, so that the upper half is electrified - on both sides, and the lower half + on both sides, the sign of the electrification being reversed as the disc passes between the combs and the armature by the electricity issuing from the combs and from the armatures. If it were not for dissipation in various ways, the electrification every where would obviously go on increasing ; but in practice a station ary condition is soon attained, in which the loss from the arma tures is just balanced by the gain owing to the action of the blunt points. After this, both sides of the disc are similarly electrified, the upper half 2 always -, the lover always + ; + electricity con tinually issuing from the points of the right comb, -electricity from the points of the left. This is, of course, accompanied by a current of + electricity from right to left through DE. The machine of Holtz, as we have described it, is somewhat uncertain in its action in our moist climate ; but a slight modifica tion of it gives excellent results. Upon the axis X is fixed a disc of ebonite, large enough just to reach between the paper armatures. This disc is fitted with a small rubber attached to the frame of the apparatus, and forms a small electric machine, which keeps the armatures continually electrified. 3 The whole is inclosed in a glass case, with a beaker of sulphuric acid to dry the air. There is a machine of this kind at present in the Cavendish laboratory at Cambridge, which never fails when the auxiliary apparatus is at all in good order. A very remarkable phenomenon often occurs when the electrodes of Holtz s machine are in connection with the armatures of a con denser of considerable capacity, and are so far separated that a spark does not pass. The machine charges the condenser up to a certain point, and then the condenser discharges along the surface of the disc. If the experiment be conducted in a dark room, a flash of light will be seen to pass along the surface of the disc, and thereafter it will be observed that the long positive brushes have shifted from one comb to the other ; after a little the condenser discharges again, and the brushes will now be seen in their old place, and so on. This phenomenon, though interesting to study, is often inconvenient in practice. To prevent it, Holtz introduced the diagonal conductor which is seen on many machines. For an account of this, and for other details concerning these machines, we refer the reader to Mascart, t. ii. 847 sqq., whose account of the more obvious principles of this apparatus is among the most lucid we have seen. His account of the experiment of causing one Holtz s machine in action to turn the disc of another by the electri cal reaction is of peculiar interest. Electromagnetic Induction Machines. The type of these is the induction coil or inductorium, sometimes called RuhmkorfFs coil, after the great Parisian instrument-maker who first brought the in strument to perfection. The object of such machines is to obtain great electromotive force from sources which furnish large quan tities of electricity, but have only small electromotive force. The principles on which the action is founded has been sufficiently indicated above in our section on the induction of electric currents. We have also given in the Historical Sketch (p. 12) some notices of the literature of the subject ; a brief enumeration of the essential parts of the machine is all that is necessary here. We have first the primary coil of thick wire and few windings, so as to have a small resistance and a small coefficient of self induc tion ; the secondary coil surrounding the primary is of thin wire (iVmm. or so), with many windings, the length in large machines being often 100,000 metres. In order to avoid the danger of dis ruptive discharge between parts of the insulated wire, the coil is divided up by insulating septa, so that parts at very different poten tials are separated. In the centre of the primary is placed a bundle of iron wires ; this greatly strengthens the action, and a good deal depends on the quality of the iron, which should be very soft. The interrupter is simply a lever, worked by the coil itself or by an electromagnet separate from the coil, by means of which the circuit
- We speak of resistances of 1 to 10,000 or 100,000 ohms.
- The line of division is not horizontal, however, if, indeed, it be
exact y a diameter. See Mascart.
- Compare Carre s machine, Masnart, t. ii. S56.
of the primary is made and broken automatically. A variety of forms have been given to the part of the apparatus ; the interrupter of Foueault is a very common one. 4 For some purposes a break driven by clock-work is used. The condenser, a very important part of the apparatus, is made of a number of sheets of tinfoil, interleaved with sheets of oiled silk or varnished paper. One set of leaves of the condenser is connected with one side of the break, and the alternate set with the other side. The function of the con denser is to provide a way for the electricity when the circuit is broken, and tnus to prevent the intense spark of the extra current in the primary, which destroys the contact surfaces of the break, and, what is worse, prolongs the fall of the primary current, and thereby reduces the average electromotive force of the induction current. Other devices have been tried for effecting the same object as the condenser, such as inserting a fine metallic wire or an electrolyte as an alternative circuit to the break ; and these answer the purpose to a considerable extent. An important improvement affecting this part of the apparatus has recently been introduced by breaking the primary circuit between the poles of a magnet, the effect of which is that the spark is suddenly drawn aside (blown out as it were). A considerable increase of striking distance between the poles of the secondary results from this arrangement. ABSOLUTE MEASUREMENTS. We have already indicated the considerations which determine the fundamental units in the two systems that have come into prac tical use. We ought now to explain how practical standards can be constructed to represent these fundamental units, or at least known multiples of them. It is necessary to have such standards in order that we may be able to measure electrical quantities in absolute measure by simple and expeditious methods of comparison, it being obviously impossible in practice to make absolute measure ments directly on all occasions. Elcctrostatical System. By means of Thomson s absolute electro- Measure meter we can determine any electromotive force in absolute measure, of In this way Thomson found the eleetromotive force of Daniell s E. M. F. battery to be 00374 C.G.S. electrostatical units. 5 By using the absolute electrometer (see art. ELECTROMETER), or Resist- another that had been compared with it, we could by the method ance. given above, p. 46, find a resistance (which was large enough to suit the method) in electrostatical measure. Then, having standards of electromotive force and resistance, we Current, could easily measure a current in electrostatic measure by applying Ohm s law. The same thing might be done by constructing the standard of quantity, which is the charge on an isolated sphere of unit radius charged to unit potential. By comparing the throw of a galvanometer when unit quantity is discharged through it with the deflection produced by any current, we could determine the latter in absolute measure by observing the time of oscillation of the galvanometer and the logarithmic decrement of its oscillation (see Maxwell, vol. ii. 749). Among the absolute measurements in the present system of units, Di- we must not omit to mention Sir Wm. Thomson s determinations electric of the dielectric strength of different thicknesses of air. From strength, these, and from the measurement of the electromotive force of Daniell s cell just mentioned, he concluded that a Daniell s battery of 5510 elements would be competent to produce a spark between two slightly curved metallic surfaces at -g of a centimetre asunder in ordinary atmospheric air. 6 Electromagnetic System. The great majority of the absolute Electro- determinations hitherto made have reference to this system. We magnetic make no attempt here to instruct the reader concerning the details of measure, this subject; such an attempt would lead us into technical particulars intelligible only to a few scientific men. We are fortunate, however, in being able to refer the English reader to two books which contain in a collected form all, or nearly all, the requisite information, viz. Maxwell s Electricity ami Magnetism, and the collected Reports of the Committee of the British Association on Electrical Standards. 7 As a specimen of the theoretical considerations involved, the Resist- reader may take Maxwell s method for determining the coefficient ance. of self-induction of a coil (given alx>ve, p. 80). If we know the value of L (in centimetres) from calculation, then equation (33) might be used to find x in absolute measure. This would not be a practicable method, inasmuch as the calculation of L would be ifticult if not impossible ; we might, however, determine L by comparison 8 with a coefficient of mutual induction which could be calculated. The earliest absolute measurement- of the resistance of a wire (by Kirch- hofl 4 See Wiedemann s Galv., or Du Moncel, Notice sur T Appareil de, llukmkorff. 8 Reprint of Papers, 305, &c. e Reprint of Papers, 340. 7 Such as wish to go deeply into the matter must read the Maas- bestimmungen of Weber.
8 Maxwell, vol. ii. 756.