Oct. 6, 1881] 



NATURE 



533 



THE INTERNATIONAL EXHIBITION AND 

 CONGRESS OF ELECTRICITY A T PARIS 1 



II. 



THE most crowded place in the Exhibition is the 

 Thedtre de I'Opdra. Here from eight to eleven on 

 three evenings in the week are to be seen four long queues 

 waiting for their turn to enter one of the four rooms where 

 the mysterious music is to be heard. Round the walls of 

 each room are hung telephones in pairs, some twenty 

 pairs in all, and the same number of persons are admitted. 

 On putting the telephones to your ears you hear the music 

 which is being performed at the opera-house more than a 

 mile distant. Some of the singers seem to be on your 

 right hand, others on your left, and it sometimes happens 

 that a particular voice is quite piercing in its loudness. 

 There are in fact ten transmitters disposed along the 

 front of the stage, near the footlights, and ten wires 

 leading from them, two of which arej connected with 

 the telephones intended for your two ears. Special 

 precautions are taken to prevent the action of the 

 transmitters from being disturbed by the tremors of 

 the boards under the feet of the actors, the transmitters 

 being supported on india rubber and loaded with lead. 

 The telephonic apparatus employed is that of the Ader 

 system. 



The greatest novelty as regards principle is exhibited 

 in Dolbear's telephone, in the United States department. 

 The receiver has no magnet, but has two parallel metallic 

 plates near together, and electrically insulated from each 

 other. One of them is connected with the line wire, and 

 the other (in the specimen here exhibited) with the return 

 wire. These two wires are connected with the terminals 

 of the secondary coil of a small Ruhmkorff at the sending 

 station ; and the voice of the speaker produces variations 

 in the primary current, on the usual plan of varying the 

 resistance in the circuit of a local battery by variations 

 of pressure. The secondary circuit is not completed, 

 inasmuch as the two plates do not touch ; but the oppo- 

 site electricities which are transmitted to them attract 

 each other on electrostatic principles, and the plates are 

 thus made to vibrate in unison with the voice of the 

 speaker at the sending station. The instrument exhibited 

 is very effective, and reproduces a whisper with greatly 

 increased intensity. It is claimed that this invention 

 does away with the disturbance experienced in other 

 telephones from currents in neighbouring wires, inasmuch 

 as such currents will not afl'ect the attraction between the 

 plates. We should add that the instrument exhibited 

 speaks fairly even when the plate next the ear is discon- 

 nected from the wire intended for it, but of course less 

 loudly than when the connection is made. This is just 

 what one would expect from electrostatic attraction, 

 the attraction of a charged for an uncharged body 

 being less than that between two bodies oppositely 

 charged. 



We have had an opportunity of seeing the system 

 adopted by Mr. Edison for the measurement of the 

 quantity of electricity consumed in each house which 

 receives a supply from one of his mains. A definite pro- 

 portion (one thousandth part) of the whole current which 

 goes through the house is shunted through a cell containing 

 two copper plates in a solution of sulphate of copper. The 

 positive plate loses, and the negative plate gains, an 

 amount of copper exactly proportional to the quantity of 

 electriiiity which passes. There are two such cells in 

 series, one serving as a check upon the other, and the 

 whole arrangement is kept under lock and key, to be 

 opened only by IMr. Edison's agents when they come 

 round to inspect the meters. As the lamps supplied (of 

 a given type) are almost precisely alike in their resistance, 



* Continued from p. 512. 



and the current, when flowing, is always nearly the same, 

 this arrangement gives a practically accurate measure of 

 the illuminating power supphed. 



Much interest has been excited by the exhibition of 

 three magneto-electric machines constructed by Prof. 

 Pacinotti of the University of Cagliari. One of these, 

 constructed at Pisa in i860, is the earliest example of the 

 principle of the ring-shaped armature, since embodied in 

 the machines of Gramme and Brush. It was originally 

 constructed as an engine to be driven by a current from 

 without ; but it was also used as a generator of electricity, 

 and both these uses of it were described in a paper in the 

 Nuovo Cimento in 1864. The machine contains an iron 

 ring like an anchor ring, round successive portions of 

 which are wound coils of insulated copper wire in de- 

 pressions cut in the ring to receive them. The inter- 

 vening portions of the ring are thus (as in the Brush 

 machine) enabled to come very nearly into contact with 

 the surrounding fixed magnets. These consist of two half 

 rings which are the pole pieces of two straight electro- 

 magnets. The coils above mentioned are connected in a 

 series, and their junctions are in connection with the 

 several segments of a commutator, as in the Gramme 

 machine. 



The second machine was constructed in 1873, and de- 

 scribed in the Nuovo Cimciito in 1874. It is a generator 

 of electricity, of the kind now known as the shunt 

 dynamo, that is to say, the current generated is divided 

 in parallel circuit between the fixed electro-magnet and 

 the external resistance. This is done by means of two 

 pairs of brushes making contact with different sections 

 of the revolving commutator. The ring is replaced by a 

 flat cylinder, across which the successive coils are wound 

 in depressions made for the purpose, the directions of 

 winding being the same as in Siemens' continuous cur- 

 rent machine, which was invented about the same time. 

 The connections of the successive coils with one another 

 and with the segments of the commutator are the same 

 as in the first machine. 



The third machine, which was constructed in 1878 on 

 a model dating from 1875, is of a type of which, so far as 

 we know, it is the only example. The idea of it is taken 

 from the well-known experiment (Arago's rotations) in 

 which a revolving horizontal copper disk causes a large 

 magnetised needle b.ilanced above it to revolve in the 

 same direction. The explanation of the effect was 

 first given by Faraday. It depends on the action 

 of a current generated in the copper disk by its 

 motion in the magnetic field due to the needle. The 

 strongest current flows along that diameter which is 

 parallel to the needle, and the current is completed 

 through the circumferential portions of the disk. Pacinotti 

 virtually cuts away all except the diametral portion and 

 one of the two circumferential portions ; in other words, 

 he takes a wire and bends it into the shape of the letter 



Q. This is one convolution of his revolving coil ; the 



next is like the same Q tilted a little ; the next is tilted a 



little more, and so on ; so that some of the convolutions 

 have the positions — 



D 



C3 



Q 



D 



the straight part of the wire passing through or nearly 

 through the axis of the coil, and the curved part being in 

 the circumference. There is no room for a core in the 

 ordinary sense, as the wires occupy nearly the whole 

 interior space ; but pieces of iron arc so disposed partly 

 within and partly without the coil as to serve the purpose 

 of a core, by increasing the induction of the fixed 

 magnets. 



{To be continued.) 



