ELECTRO-MAGNETISM. 



acquainted with the details we have 

 already given of analogous experiments. 

 It will be sufficient to observe that there 

 are, in general, two kinds of rotation of 

 which the moveable parts of the appa- 

 ratus are susceptible, the one on a verti- 

 cal axis, as in figures 66, 71, 84, 91, 92, 

 93, and the other on a horizontal axis, 

 as in figures 85 and 96. 



(193.) Care should always betaken, 

 in nice experiments, to guard against the 

 errors that might arise from allowing 

 the influence of the earth to interfere 

 with the actions we are examining. This 

 disturbing force may, in general, be 

 neutralized, and rendered ineffective by 

 particular dispositions of the conducting 

 wires. Thus the moveable conductor, 

 fig. 92, may be rendered astatic or inde- 

 pendent of terrestrial influence, by form- 

 ing a second parallelogram below the 

 first, composed of wires, so turned as to 

 oblige the current to pass in opposite 

 directions in corresponding parts of each. 

 This is shown in Jig. 1 05, in which P 

 and N represent the'steel points affixed 

 to the extremities of the wires for the 

 purpose of being placed in cups with 

 mercury ; and the directions of the cur- 

 rents in the several portions of the wire 

 are indicated by arrows, whereby it 

 appears that the action of the earth 

 upon any one part is neutralized by its 

 equal and opposite effect in another cor- 



Fig. 105. v 

 -UP 





responding part, similarly situated with 

 regard to the axis of rotation. The 

 wires should'be covered with silk thread, 

 in order to prevent metallic contact in 

 the parts where they are brought to- 

 gether. The shaded parts represent the 

 branches that may be conveniently tied, 

 or simply twisted together, after this 

 precaution has been taken, for the sake 

 of greater firmness. The weight W is 

 applied as a counterpoise on the other 

 side of the axis of rotation. 



Fig. 107. 



(194.) Fig, 106 represents another 

 form of an astatic conductor, having the 

 same properties as the preceding, but 

 better adapted for the examination of 

 the actions on the lower horizontal 

 branch. 



(195.) Fig. 1 7 shows an arrangement 

 of a similar kind, in which the horizon- 

 tal branches neutralize one another, but 



in which the interior vertical branches, 

 being in the axis of motion, have no 

 influence in producing rotation ; and the 

 exterior vertical branches are therefore 

 uncompensated, as far as relates to the 

 action of any current presented to them: 

 although being in opposite sides of the 

 axis, they neutralize one another as far 

 as regards the influence of the earth. 



