Lifting Power of the Electro-Magnet. 299 



nication,* as including a wider range, and being independent of the intensity of 

 its needle's magnetism. Its circular conductor consists of five copper rings, 

 each 0'5 broad and 005 thick, the innermost of which has 16 inches internal 

 diameter. This is commonly used alone, but the others can be combined with 

 it. The connectors descend from the nadir of the rings within the wooden 

 stem which supports them, pass through its base (which is provided with 

 levelling screws), and then, proceeding about 18 inches in the magnetic meri- 

 dian, turn at right angles, and proceed parallel, and almost in contact, for three 

 feet, to a commutator which connects them with the general circuit. By thus 

 reversing the current, not merely in the rheometer, but also in so great a length 

 of the connectors, I designed to eliminate their influence ; and experience shows 

 that such a precaution is quite necessary. Concentric with the rings, and per- 

 pendicular to their plane, is fixed a brass circle 9 inches diameter, divided to 

 half degrees, at whose centre stands a point of hard steel, very carefully 

 finished to an angle of 60".! On this turns a needle 1'.77 long, 0'.25 deep, and 

 0'05 thick ; it has a ruby cap, and pointers of palladium long enough to reach 

 the divisions ; and it weighs altogether 75 grains. It has been shown by 

 Weber (Poggendorf, vol. Iv.) that if the ratio of the ring's diameter to the 

 length of the needle be greater than 4 or 5, the tangent of deflection is propor- 

 tional to the force. This ratio, however, is too low for high deflections. When 

 it is 4 I find the law fails at 33°, and when 4-8 at 50. In this rheometer it is 9. 

 As, however, it was necessary to ascertain whether the influence of the con- 

 nectors was injurious, I at the same time examined its sufficiency by the volta- 

 meter, and found for 28 angles from 20"^ to 16",% that the tangents are exactly as the 

 quantities of mixed gases evolved in a given time, supposed dry, and at the nor- 

 mal temperature and pressure. The factor by which the tangent gives the current 

 force i^ depends on the unit assumed for that quantity. Weber, in the memoir 

 referred to, uses one derived from the intensity of terrestrial magnetism at the 



* Transactions of the Royal Irish Academy, vol. xxi. p. 303. 



■f It was formed by traversing it while rapidly revolving in the drill apparatus of a slide-rest, 

 inclined at 30° along the surface of a cyliudric lap also rapidly revolving, and charged first with 

 very fine emery, and then with crocus. It bears examining with a power of 1 20 diameters, and is 

 far more perfect than any point which I have seen in a theodolite or compass. 



% The greatest which 18 Groves' could produce with the voltameter. 

 VOL. XXII. 2 R 



