1875.] of Bar-magnets and Conducting -wires. 123 



4. Upon a thin wooden tube 15 centims. long and 7 millirus. bore I 

 wound a cotton-covered copper wire 1*7 millim. diameter from one 

 end of the tube to the middle, then reversed the direction of winding 

 and continued to the other end and back to the middle, again reversed 

 and coiled to the first end of the tube; by which arrangement the 

 passage of a current through the coils produced two similar poles at the 

 ends of the tube, and two others of the opposite kind at the middle. 



The tube being now fixed in a vertical position, a straight iron wire 

 15 centims. long and 1*8 millim. diameter, pointed at its lower end and 

 surmounted by a brass mercury- cup 5 millims. diameter, containing a 

 drop of mercury, was supported entirely within the tube and free to 

 rotate, by a similar cup (surmounting a fixed vertical brass rod), at the 

 lower end of the tube, the upper end of the axial wire being kept in 

 position by a vertical brass rod fixed above the coil and terminated at its 

 lower end by a sharp point of platinum in the mercury-cup. 



A current from 6 one-pint Grove's elements arranged as 3 being 

 now passed through the coil, brass rods, and axial wire, the latter rotated 

 rapidly. 



With a downward current through the axial wire, and north poles * at 

 the ends of the tube, the upper end of the wire rotated in the same 

 direction as the hands of a watch. Reversing the direction of the 

 current either in the coil or iron rod, reA r ersed of course the direction of 

 motion. The rotation in this case was not due to obliquity of coil- 

 current, because that was neutralized by the second layer of coils ; nor to 

 portions of transverse currents proceeding to or from the brass rods, 

 because each of those portions was 10 centims. distant from the ends of 

 the axial wire ; nor was it due to the portions of current entering or 

 leaving the coil, because they entered and left at the same part and in 

 parallel directions, and thus neutralized each other's effect. A copper wire 

 substituted for the iron one would not rotate, probably because copper is 

 so little capable of acquiring longitudinal magnetism. 



5. To ascertain if the rotation was merely due to an action of the 

 current in the coil upon either the axial current or longitudinal magnetism 

 of the iron, or whether the coil-current simply performed the function of 

 longitudinally magnetizing the axial wire, I took an iron wire 23 centims. 

 long and 2*7 millims. diameter, sharp-pointed at its lower end, soldered 

 to its upper end a double wire of cotton-covered copper, each wire being 

 1*7 millim. diameter, coiled the double wire upon the axial rod in two 

 layers (so as to enable two similar poles to be formed at the extremities 

 of the axis), and terminated the copper wires by a little brass mercury-cup 



* By a North pole I mean that which points to the South, and by a South one that 

 which seeks the North. Because if we call that end of a magnet the north which is 

 repelled by the south pole of the earth and attracted by the north pole, we call by dis- 

 similar and opposite names two things which are essentially alike, and by similar 

 names two things which are essentially different and opposite. 



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