ELECTRICITY. 



189 



a \er\ feeble current generated in it; thi- 



>:i;iI tin- elect ro-ina::net., will iiirre.-i-e its 

 :-y ;iil(litii>!i:il impulse. It will 



thus be teen that tin- <ni\ limit to tin- power 



bine i- the rapidity with which the 

 ll made to rotate, which is entirely 

 tin- amount of dynamic, force 

 employed. But the invat improvement in this 

 ino is the introduction of the second nrmn- 

 which, although it takes off very power- 

 ful currents, generated in its wire by the in- 

 !_rnctiMn. does not at all interfere 

 wi;h the primary current of the electro-magnet. 

 Tlie m.ieiiinf placed by the inventor in the 

 Kxhihition measured about 24 in. in 

 :, \-2 in. in width, and 7 in. high; but 

 i.eiiig im|)erfcctly constructed as to its 

 proportions, the results obtained were prob- 

 aMy, no doubt, ranch less than they would bo 

 with a properly-constructed machine. Still it 

 i'.umd that the machine would keep 50 in. 

 of platinum wire, .10 in. diameter, incandescent, 

 and when, a small voltameter was placed in 

 circuit with the second armature it would give 

 off 250 cubic centimetres of gas per minute, 

 and in connection with an electric regulator 

 would give a light equal to about thirty -five 

 Grove's or Bunson'a elements, the driving 

 power expended being less than one horse. 



Mr. Ladd then proceeded to describe a ma- 

 chine on the same principle as that just noticed, 

 but which, instead of having two independent 

 armatures running in separate grooves, has two 

 armatures fixed end to end, so as to appear like 

 one continuous armature, but so placed with 

 reference to each other that their magnetic axes 

 shall be at right angles. By this arrangement 

 there is only one opening required for the 

 armature, enabling us to take full advantage of 

 the horse-shoe form of electro-magnet. The 

 shoes of the electro-magnet and armatures are 

 so proportioned to each other that there is an 

 actual break in the magnetic circuit with refer- 

 ence to each armature alternately, but by their 

 disposition at right angles there never is an 

 actual break in the complete magnetic circuit, 

 but simply a shifting of the principal portion 

 of the magnetic force from one armature to the 

 other at the precise moment required to pro- 

 duce the best effect. The mechanical advan- 

 tages obtained by this disposition of parts mu>t 

 be at once obvious, as one pair of bearings and 

 set of driving-gear is dispensed with, and from 

 the fixing of the two armatures together the 

 currents aro made to flow perfectly isochro- 

 nously. It may be found of advantage to vary 

 the angle of position of the armatures with ref- 

 erence to each other, according to the speed at 

 which they are driven, so that the current given 

 off by the exciting armature may at the precise 

 moment exert its full effect upon the electro- 

 magnet, and thus produce the best effect in the 

 second armature. -(Philosophical Magazine.) 



The Mr-trirfl Condition of the J?arth.M. 

 N. de la Rive furnished to Comptes Rendus, in 

 Juno, a report of laboratory experiments made 

 VOL. TH. 19 



by him to determine the electrical condition of 

 the globe. IK; placed as an insulating support 

 a sphere of about 30 centim u-U-r, 



made of porous earthenware, or of wood cov- 

 ered with bibulous paper, so as to have, by 

 wetting the surface, a moist condition repre- 

 senting the earth. To the highest part of this 

 sphere, in contact with its moist surface, he 

 .; small metal disk; a second one was 

 arranged in the same manner at a distance of 

 from 50 t,o 90 degrees from the first, lie then 

 joined the two disks by the wire of a galvanom- 

 eter. No current showed- itself, either posi- 

 tively or negatively, even when the insulated 

 sphere was positively or negatively electrified. 

 By means of an insulated coil he then sus- 

 pended, at a distance of 2 to a centimetres 

 above the sphere, a plate slightly concave on 

 the lower side, and of such dimensions that it 

 only covered a small portion of the sphere 

 that, namely, in the midst of which was the 

 upper metal disk, and therefore not the portion 

 in which the other disk was placed. The ap- 

 paratus being thus arranged, he charged the 

 sphere representing the earth with negative 

 electricity from a machine, the positive elec- 

 tricity of which was led to the concave metal- 

 lic surface representing the atmosphere. The 

 galvanometer quickly indicated the existence 

 of a current, the direction of which was from 

 the lower to the upper disk. This current was 

 perfectly regular, and lasted as long as the 

 machine was at work. It is to be observed 

 that the upper part of the disk was in that part 

 of the sphere where the most negative elec- 

 tricity was accumulated, under the influence 

 of the insulated positive plate ; while the second 

 disk was in the part withdrawn from the influ- 

 ence that, therefore, in which the quantity 

 of negative electricity was small, and flowed 

 out in proportion as it was produced, into the 

 surrounding air. The current proceeded, 

 therefore, in the wire which joined the two 

 unequally electrified portions of the negative 

 sphere, from the least electrified portion to that 

 which was more so. This part of the experi- 

 ment exactly agreed with the phenomenon 

 observed by M. Matteucci, viz., that in every 

 mixed circuit consisting of a layer of earth and 

 a metal wire, the ends of which are sunk in 

 the ground, minute precautions being taken to 

 avoid any thermal or chemical action, the wire 

 is traversed by an electrical current, whose 

 direction is constant whenever the soils in 

 which the ends are placed are of unequal 

 heights. The current ascends on the metal 

 wire, its intensity increases with the length of 

 the wire, and with the difference in the level 

 of the ends. When M. de la Rive produced a 

 series of discharges by bringing the positive 

 plate too near the negative sphere below, he 

 saw the needle of the galvanometer deflect, 

 sometimes in one, sometimes in the other direc- 

 tion, making very irregular movements, instead 

 of preserving the constant deviation which it 

 experienced when there was no discharge. 



