438 Dr. G. F. Barker on a new 



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 referred to the circle 

 was 16 feet across, and the needle was 14 feet long, the field 

 being brilliant. 



The method of construction which has now been described is 

 evidently capable of producing a galvanometer for demonstra- 

 tion 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 mag- 

 netism, 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. 



Secondly, the space beneath the mirror is sufficiently large to 

 permit the use of a coil of any needed size. Since, therefore, 

 the lower needle is entirely enclosed within the coil, the field of 

 force within which it moves may be made sensibly equal at all 

 angles of deflection, as in the galvanometers of Sir William 

 Thomson. Hence the indications of the instrument may be 

 made quantitative, at least within certain limits. The circular 

 coil, too, has decided advantages over the flat coil, since the mas3 

 of wire, being nearer t6 the needle, produces a more intense field. 

 Were it desirable, a double coil, containing an astatic combina- 

 tion, could be placed below the mirror, the upper needle in that 

 case serving only as an index. The instrument above described 

 has a coil 3 inches in diameter and 1 inch thick, the dia- 

 meter of the core being 1 inch. Since its resistance is only 

 about a quarter of an ohm, it is intended for use with circuits of 

 small resistance, such as thermo-currents and the like. 



The results of a few experiments made with this new vertical- 

 lantern galvanometer will illustrate the working of the instru- 

 ment, and will demonstrate its delicacy. The apparatus used 

 was not constructed especially for the purpose, but was a part 

 of the University collection. 



1. Induction- currents, — The galvanometer was connected with 



