ELECTRO-MAGNETISM. 



93 



of Philosophy*. A bar of antimony, 

 about eight inches Ions: and half an inch 

 square, was taken, and its extrem;tios 

 connected by twisting a piece of brass 

 wire round them so as to form a loop, 

 each end of the bar having several coils 

 of the wire. On heating one of the ex- 

 tremities, for a short time, with a spirit- 

 lamp, electro-magnetic effects were pro- 

 duced in every part of a circuit so 

 formed. 



(304.) Thus it appears that for con- 

 stituting a circuit of this kind, two ele- 

 ments only are requisite ; 'which may 

 be represented in the diagram, fig. 169, 

 by the conductors A and B, consisting 



Fig. 169. 



of two different metals, in contact in 

 two points H and C, so that a circuit is 

 formed in H A C B. 



(305.) The electrical current thus ex- 

 cited has been termed Thermo-electric, 

 in order to distinguish it from the com- 

 mon galvanic current, which, as it re- 

 quires the intervention of a fluid ele- 

 ment as one of its essential components, 

 was denominated a Hydro-electric cur- 

 rent. The term Stereo- electric current 

 has also been applied to the former, in 

 order to mark its being produced in 

 systems formed of solid bodies alone. 

 It is evident that if, as is supposed in 

 the theory of Ampere, magnets owe 

 their peculiar properties to the continual 

 circulation of electric currents in their 

 minute parts, these currents will come 

 under the description of stereo-electric 

 currents. 



(306.) The chief evidence we possess 

 of the existence of thermo-electric cur- 

 rents consists in the production of elec- 

 tro-magnetic effects. A compass-needle 

 placed either within or without the cir- 

 cuit, and at a small distance from it, is 

 deflected from its natural position in a 

 direction conformable to its situation 

 with regard to the circuit. Still stronger 

 indications of electro-magnetic action 

 are obtained by placing two ends of one 



New Series, vol. iv., p. 318. 



of the metallic arcs in contact with the 

 wires of a galvanometer. The thermo- 

 electric current has also been found to 

 excite contractions in the muscles of a 

 frog : but as far as experiments have 

 yet been tried, it is inadequate to effect 

 chemical decompositions, the ignition of 

 metals, or to exhibit sparks, or any 

 other of the phenomena of ordinary 

 electricity. 



(307.) If the metallic arcs, through 

 which a thermo-electric current is made 

 to pass by the application of heat to one 

 of the points of contact of the different 

 metals, be delicately suspended, they will 

 obey the action of a magnet brought 

 near it. If the opposite poles of two 

 magnets be placed on the outside of a 

 circuit moving in a vertical plane, and 

 turning on a vertical axis, the conduc- 

 tors may be made to revolve by continu- 

 ing to apply the heat on the same side. 

 Thus if the circular arrangement of bars 

 represented infg. 169 be suspended by 

 a thread at A, and opposite magnetic 

 poles be applied at H and C, out of the 

 circle, while the flame of a spirit-lamp 

 is held steadily at H, the combined ac- 

 tions of the two magnetic poles upon 

 the adjoining ascending current at H, 

 and descending current at C, will be to 

 move the circle till its plane is at right 

 angles to its former position. But the 

 impulse it has acquired by the joint ac- 

 tion of the magnets is sufficient to con- 

 tinue the motion until the side C arrives 

 at the flame. This part of the circle 

 being thus heated, while the part H is 

 at the same time becoming cool, an 

 electric current is now determined from 

 G through A to H, and back again 

 through B to C, which direction, with 

 reference to the magnets, is the same as 

 before, and the circle is urged onwards 

 in its revolution. When it has com- 

 pleted an entire revolution, all the cir- 

 cumstances being the same as at first, 

 another impulse will be given, and the 

 circle will continue incessantly to re- 

 volve ; the current moving alternately 

 in opposite directions at every semi-re- 

 volution of the circle. 



(308.) On the other hand, the pole 

 of a magnet placed within the circuit 

 will have no tendency to produce rota- 

 tion ; because the current in the oppo- 

 site branches of the circle moves in con- 

 trary directions; and being, therefore, 

 urged by the magnet to revolve in oppo- 

 site directions, the circle will remain in 

 equilibrium. But two systems of circles 

 supported each by a point on the ends 



