470 Royal Society : — 



fact, but also because it provides a simple means of producing very 

 powerful electrical effects. 



The apparatus employed in this experiment is an electromagnetic 

 machine consisting of one or more horseshoes of soft iron surrounded 

 with insulated wire in the usual manner, of a rotating keeper of 

 soft iron surrounded also with an insulated wire, and of a commu- 

 tator connecting the respective coils in the manner of a magneto-elec- 

 trical machine. If a galvanic battery were connected with this ar- 

 rangement, rotation of the keeper in a given direction would ensue. 

 If the battery were excluded from the circuit and rotation imparted 

 to the keeper in the opposite direction to that resulting from the 

 galvanic current, there would be no electrical effect produced, sup- 

 posing the electromagnets were absolutely free of magnetism ; but 

 by inserting a battery of a single cell in the circuit, a certain magnetic 

 condition would be set up, causing similar electromagnetic poles to 

 be forcibly approached to each other, and dissimilar poles to be 

 forcibly severed, alternately, the rotation being contrary in direction 

 to that which would be produced by the exciting current. 



Each forcible approach of similar poles must augment the magnetic 

 tension and consequently increase the power of the circulating cur- 

 rent ; the resistance of the keeper to the rotation must also increase 

 at every step until it reaches a maximum, imposed by the available 

 force and the conductivity of the wires employed. 



The cooperation of the battery is only necessary for a moment of 

 time after the rotation has commenced, in order to introduce the 

 magnetic action, which will thereupon continue to accumulate without 

 its aid. 



With the rotation the current ceases ; and if, upon restarting the 

 machine, the battery is connected with the circuit for a moment 

 of time with its poles reversed, then the direction of the continuous 

 current produced by the machine will also be the reverse of what it 

 was before. 



Instead of employing a battery to commence the accumulative 

 action of the machine, ii suffices to touch the soft-iron bars employed 

 with a permanent magnet, or to dip the former into a position pa- 

 rallel to the magnetic axis of the earth, in order to produce the 

 same phenomenon as before. Practically it is not even necessary 

 to give any external impulse upon restarting the machine, the 

 residuary magnetism of the electromagnetic arrangements employed 

 being found sufficient for that purpose. 



The mechanical arrangement best suited for the production of these 

 currents is that originally proposed by Dr. Werner Siemens in 1857* 

 consisting of a cylindrical keeper hollowed at two sides for the recep- 

 tion of insulated wire wound longitudinally, which is made to rotate 

 between the poles of a series of permanent magnets, which latter are 

 at present replaced by electro-magnets. On imparting rotation to 

 the armature ' of such an arrangement, the mechanical resistance is 

 found to increase rapidly, to such an extent that either the driving- 



. . * See Du Moncel ' Sur l'ElectriciteV 1862, page 248. 



