THE PROPAGATION OF ELECTRICITY. 187 



about fifteen centimetres in diameter, furnislied with four tubulures situated at 

 the respective extremities of two diameters of the globe, which intersect one 

 another at right angles. Two cylindrical rods of soft iron are fixed by means 

 of two of these tubulures in the interior of the globe, in the direction of 

 the same diameter, so that their interior extremities may be at a distance of 

 about eight or ten centimetres from one another, while their exterior extremities 

 project from the tubulure nearly two centimetres. It is these exterior extremi- 

 ties which are to be placed in contact with the poles of a strong electro-magnet, 

 in order that the interior extremities may thus become two magnetic poles. 

 The two other tubulures serve to introduce into the interior of the globe two 

 isolated metallic rods, terminated by balls Avhich are at a distance of about ten 

 centimetres from one another, and which serve as electrodes to the electric 

 stream whose direction is thus equatorial, that is to say, perpendicular to the 

 right line which joins the two magnetic poles. As long as the rods of soft iron 

 are not magnetized, the electric stream remains perfectly rectilinear ; but so soon 

 as magnetization takes place, the stream, which we will suppose to have a hori- 

 zontal direction, takes the form of a half circumference of a circle situated either 

 above or below the line which joins the magnetic poles, according to the direc- 

 tion of the magnetization or that of the discharge. The form of the luminous 

 arc is that of a half ring much flattened, as well as widened. The striae are 

 strongly marked in it, more than they were in the rectilinear current, and its 

 exterior part is much serrated, especially when the gas contains a little vapor 

 of alcohol or ether. If the electric current, instead of being equatorial, is axial, 

 that is to say, directed from one of the magnetic poles to the other, these two 

 poles serving it as electrodes, it exijerieuces no sensible modification under the 

 influence of magnetization. 



If, however, the discharge is made to pass between a ball of brass and one 

 of iron, placed at the extremity of an iron rod so as to be cajDable of being mag- 

 netized, there is observed, at the moment of magnetization, a movement of de- 

 pression, or of elevation in the luminous atmosphere which surrounds the ball 

 of iron. This movement pertains evidently to the change of direction under- 

 gone by the electric filaments which radiate from the ball. But the best mode of 

 studying the action of magnetism in the cases where the magnetized bar is in the 

 interior of the gas, is to make use of a bell or cylindrical jar sixteen centimetres 

 in diameter by twenty centimetres in height, in the axis of which is placed a 

 rod of soft iron having a diameter of about three centimetres, whose rounded 

 end is situated at the middle of the axis of the cylinder. This rod is planted 

 in a circular disc, which serves to close the jar. A metallic ring, about twelve 

 centimetres in diameter, formed of Avire from 3 to 4"^™ in diameter, and having 

 for its centre the top of the iron rod, is situated in a plane perpendicular to the 

 axis of the jar. This ring communicates, by means of a rod covered with an 

 isolating coat which is soldered to it, with one of the poles of the Ruhmkorff" 

 apparatus, Avhile the other pole is placed in communication, outside the jar, 

 with the extremity of the rod of soft iron, which, in the interior of the jar, is 

 also covered with an isolating coat, except at its summit. It is between this 

 summit and the ring of which it is the centre that the discharge takes place. 

 In order to magnetize the rod of soft iron, it now sufiices to place it in contact, 

 by its exterior extremity, with the pole of an electro-magnet, taking care to 

 place between the two a thin strip of caoutchouc to serve as an isolating layer, so 

 that the whole apparatus shall be well isolated. The cylindrical jar is also 

 closed at that one of its two extremities where the rod of soft iron is absent, 

 and it is there furnished with two cocks, of which one serves to form a vacuum, 

 and to introduce a gas which is more or less rarefied ; and the other, constructed 

 in Gay Lussac's manner, permits the introduction into the ball of a greater or 

 less quantity of vapor of whatever nature. 



