1823.] Prof . Barlow on Magnetic Attractions. 457 



needle by the action of a galvanic wire, might be explained by 

 the admission of one simple principle ; viz. that every particle of 

 the galvanic fluid in the conducting wire acts on evert/ particle 

 of the magnetic JIuid in a magnetized needle, with a force varying 

 inversely as the square of the distance ; but that the action of the 

 particles of the fluid in the wire is neither to attract nor to repel 

 either poles of a magnetic particle, but a tangential force which 

 has a tendency to place the poles of either fluids at right angles to 

 those of the other ; whereby a magnetic particle, supposing it under 

 the influence of the wire only, would alivai/s place itself at right 

 angles to the tine let fall from it perpendicular to the wire, and to 

 the direction of the wire itself at that point." 



" I pretend not to illustrate the mechanical principles by 

 which such an action can be produced ; I propose only to show, 

 that if such a force be admitted, all the results obtained from the 

 reciprocal action of a galvanic wire and a magnetized needle 

 may not only be explained, but computed, and that the results 

 agree numerically with experiments." 



The galvanic instrument employed by Mr. Barlow differs 

 from Dr. Hare's calorimotor merely in the mechanical contrivance 

 for lowering it into and raising it out of the fluid ; " that part of 

 the apparatus which peculiarly appertains to the experiments 

 I am about to detail," he says, " is represented in fig. 10, 

 A B is an upright stand, placed near the poles of the battery ; 

 a b, c d, are two staples of stout copper wire, driven into the 

 upright, the two ends at b and c passing quite through, as 

 shown at C and Z ; and on which two wires are fastened by spiral 

 turns, and with which the communication is made with the 

 poles of the battery ; ef, g h, are two copper wires of the same 

 dimension as the staples, each four feet long, having their ends 

 flattened and drilled so as just to enable them to slide freely 

 upon the wires a b, c d, and the vertical wirefh, also four feet 

 in length, which passes through a hole in the top of the table 

 F G H I, and so tight as to render it perfectly fixed. On the 

 plane of the table, which is two feet in square, the circle N E S W 

 is described about the centre o, and divided into the points of 

 the compass and smaller divisions ; NS is an index or box 

 ruler, through which the wire fit passes, so that the former may 

 be turned freely about the latter, and set to any proposed azi- 

 muth. On this ruler is placed the small compass c' , by means 

 of which the deviation at any given time may be taken ; c" is 

 another compass placed on the top of the support L c" , and is 

 intended to remain fixed in its place, in order to serve as a 

 standard for estimating and comparing the power of the battery 

 at different times." 



" For the principal experiments, this apparatus is placed so 

 that the plane of the rectangle of wires is perpendicular to the 

 magnetic meridian ; because in this position the horizontal 

 wires being east and west, they have no effect in deflecting the 



