■46 Electro-jMegneiic Expcrimenis. 



t'xamine tlie action whicrh a inagiict exercises on the galvanic arc. 

 tfince no body can put another in motion, without being put iu 

 «iotion in its turn, when it possesses the requisite mobility, it is 

 iCasy to foretel that the magnet must move the galvanic arc. 



To mark the motion given by the magnet to a simple galvanic 

 apparatus, I employed varionis arrangements. One of these will 

 be found represented in fig. 5, which exhibits a perpendicular 

 .section of it in the direction of the breadth. A trough of copper 

 cccc is ihree inches high, four inches long, and half an inch 

 broad ; -dimensions, which may of course be vr.ried at pleasure. 

 It mav be observed, however, that tiie breadth should not be 

 great, and that the plates of the trough should be as thin as pos- 

 •oible. A plate of zinc z z, is kept in its position by two pieces of 

 cork e € ; € f J f j z is a brass wire, of at least a quarter of a line 

 in diameter ; a i& is a brass wire as fine as possible, so as to be 

 able at the same time to support the weight of the apparatus ; 

 c <7 c is a linen thread which unites the wire to tiic apparatus. 

 The fluid conductor is contained in the trough. The conducting 

 ■ wire of this apparatus will attract the north pole of the needle 

 when it is ])lac€d on the left side of the plane c fff f z that is 

 observed in the directiovi^'x. The south pole will, on the same 

 «ide, be repelled. I5ut on the other side of this plane, the north 

 pole will be repelled, and the south attracted. To ensure this ef- 

 fect, the needle must not be placed above /' /j nor below^'z or 

 f c. If, instead of presenting a small moveable needle to the 

 conducting wire, there is presented near one of the extremities 

 J f\ one of the poles of a powerful magnet, the attraction or re- 

 pulsion in4icated by the needle, will put in motion the galvanic 

 aoparatus, and turn it round the prolonged axis of a b. 



Take, instead oftheconductingwire, a strip of copper of the same 

 breadth as the zinc plate, and the only dilforence from the effect 

 ^ust mentioned, will consist in its being much feebler. The effect 

 is, on the other hand, increased a little, by making the conduc- 

 tor very short. In fig. 6. will be seen a perpendicular section of 

 this arrangement, iu the direction of the breadth of the trougli. 

 in fig. 7- the sanie arrangement may be seen iu perspective. 

 The conducting plate is represented by abed e f, and the zinc 

 plate by c z zf. The north pole of the needle will, in this ar- 

 rangement, be attracted towards the plane of a /■ c, and the south 

 will be repelled from the same ])Iane. Contrary effects will take 

 place bv an apparatus e d f, whose extremities act like the poles 

 of a needle. It must be confessed, however, that only the faces 

 of the two extremities, and not the intermediate parts, possess 

 this analogy. 



A moveable galvanic apparatus may likewise be made of two 



plates. 



