76 DESCRIPTION OF THE TORSION GALVANOMETER. 



respects the zero point. This I have always avoided, by ascertaining the zero at the 

 beginning and close of each experiment. 



256. After having had some experience with voltameters, deflecting galvanometers, 

 &c., I am induced to describe the instrument used in these experiments, for it will ena- 

 ble those who are not accustomed to the torsion balance to execute measures very 

 easily, which they might otherwise ineffectually attempt. 



257. A A, B B (fig. 28), is a glass jar, 16 inches high, open at both ends; at A A 

 it is 2i inches in diameter, at B B 6 inches ; it rests upon a piece of wood 8 inches by 

 10. A strap of stout sheet-copper, effe, I inch wide and 15 long, is bent into the 

 form indicated ; its extremities at e e being let into the wood, and bearing mercury 

 boxes, D D. The central part of this strap, from /to/, is placed horizontally, and has 

 a circular aperture and side gap, as is shown in fig. 29, a a, through which the spindle 

 carrying the needle can be passed, and works. 



258. The upper extremity of the jar, A A, is accommodated with a divided circle, in 

 the centre of which the key G works : this key is ground, like a stopcock, to a slightly 

 conical figure ; it therefore revolves very truly without any shake : it is drilled longitu- 

 dinally to admit the passage of the glass thread, which is secured in it by means of a 

 perforated straw and a drop of sealing-wax. 



259. The other extremity of the thread enters a little tubular perforation in the ivory 

 axis, n n', and is also secured therein by wax. Only one needle is used ; it is lozenge- 

 shaped, and is 4i inches long. Besides carrying this needle, the ivory axis extends an 

 inch and a half below it, and in a slit at its lower extremity, confines a parallelogram 

 of stout tinfoil, r r, an inch wide, and 2f long. When in use, this vane of tinfoil works 

 in a glass cup, k k, 85 inches in diameter, which is filled with water. 



260. One of the chief improvements in the instrument is connected with the needle, 

 and the axis on which it works. The latter is a small cylinder of ivory ; it has two flat 

 faces filed upon it, corresponding to the direction of the needle. On each of these 

 faces, as is represented in (fig. 30), is drawn a vertical line, and a little to the right of 

 it are placed five dots. The polar extremities of the needle are accommodated with 

 two upright wires, pp',pp', an inch long, which serve as indexes; and at a distance 

 of 10 or 15 inches, in the magnetic meridian, a plate of metal, not shown in the figure, 

 with a small hole in its centre, is placed, to be used as a sight. When an observation 

 is to be made, the experimenter adjusts this sight in front of the instrument, either on 

 its north or south side ; and on looking through it, as soon as the needle moves, he 

 sees the index, pp', traverse before the scale on the axis. There is no shake or vibra- 

 tion, even though any one should cross the floor or jar the table, for the index and 

 the scale equally participating in all these disturbances, the motion is almost as steady as 

 that of a shadow on a sundial ; the vane of tinfoil does not in the least interfere with 

 the accuracy of indication, but effectually stops the oscillations, and the utmost accuracy 

 may be obtained, by previously giving the index, p p', a slight bend out of the vertical 

 line, and using the five dots as a diagonal vernier. 



261. In the following memoir, it will be seen that the terms primary and secondary 

 wire are occasionally used, the former in a somewhat extended sense : I mean by it 



