LSTo.] 



443 



f Barker. 



needle is a circular coil of wire, having a cylindrical hollow core of an inch 

 in diameter, in which the needle swings, and a smaller opening transverse to 

 this, through which the suspension wire passes. In the apparatus already 

 constructed (in which the upper needle is five centimeters long,) the coil is 

 composed of 100 feet of No. 14 copper wire, and has a resistance of 2o5 

 ohm. The accompanying cross section (Fig. 2,) of the vertical-lantern 

 galvanometer as at present arranged, drawn on a 

 scale of jV, will serve to make the above descrip- 

 tion more clear. A is the needle, suspended di- 

 rectly above the scale-glass D, by a silk filament, 

 passing through the loop B, close under the obj'c- 

 tive C. This needle is attaclied to the aluminum 

 wire a b, which passes directly through the scale- 

 glass D, the condensing lens E, and the inclined 

 mirror F at H, and carries, near its lower end, the 



f^Tj^.^ second needle I, This needle is shorter, (its length 



is 2.2 centimeters,) and heavier than the upper one, 

 and moves in the core of the circular coil J, whose 

 ends connect with the screw-cups at K. This coil 

 rests on the base of the lantern, enclosed in a suit- 

 able frame. It is obvious that when the instru- 

 ment is so placed that the coil is in the plane of the 

 Fig. 2. magnetic meridian, any current passing through 



this coil will act on the lower needle, and, since both needles are at- 

 tached to the same wire, both will be simultaneously and equally deflect- 

 ed. Upon the screen is seen only the graduated circle and the upper 

 needle ; all the other parts of the apparatus are either out of the field or 

 out of focus. Moreover, the hole in the lens is covered by the middle 

 portion of the needle, and hence is not visible. The size of the image is, 

 of course, determined by the distance of the galvanometer from the 

 screen ; in class experiments, a circle 8 feet in diameter is sufficient ; 

 though in the lecture above refei-red to, the circle was 16 feet across, 

 and the needle was fourteen feet long, the field being brilliant. 



The method of construction which has now been described, is evidently 

 capable of producing a galvanometer for demonstration, whose delicacy 

 may be determined at will, depending only on the kind of work to be done 

 with it. In the first place, the needles may be made more or less perfectly 

 astatic, and so freed more or less completely from the action of the earth's 

 magnetism, and consequently more or less sensitive. Moreover, an astatic 

 system seems to be preferable to one in which damping magnets are used, 

 since it is freer from influence by local causes ; though, if desirable for 

 a coarser class of experiments, the considerable distance which separates 

 the needles in this instrument, allows the use of a damping magnet with 

 either of them. In the galvanometer now in use, the upper needle is the 

 stronger, and gives sufficient directive tendency to the system to bring 

 the deflected needle back to zero quite promptly. In the experiments 

 referred to below, the system made 25 oscillations per minute. 



