Barker.] 444 [May 7, 



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 Wm. Thomson. Hence the indications of the instru- 

 ment may be made quantitative, at least within certain limits. The cir- 

 cular coil, too, has decided advantages over the flat coil, since the mass of 

 wire being nearer to the needle, produces a more intense field. Were it 

 desirable, a double coil, containing an astatic combination could be 

 placed below the miiror, the upper needle, in that case, serving only as an 

 index. The instrument above described has a coil three inches in 

 diameter and one inch thick ; the diameter of the core being one 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 instrument, and will 

 demonstrate its delioacy. The apparatus used was not constructed 

 especially for the purpose, but was a part of the University collection. 



hiduction Gurrents. — 1. The galvanometer was connected with a coil 

 of covered copper wire. No. 11 of the American wire gauge, about ten 

 centimeters long and six in diameter, having a i-esistance of 0. 323 ohm. 

 A small bar magnet, 5 centimeters long and weighing six and a-half 

 grams, gave, when introduced into the coil, a deflection of 40°. On with- 

 drawing the magnet the needle moved 40° in the opposite direction. 



2. A small coil, 20 centimeters long and 3.5 in diameter, made of No. 16 

 wire and having a resistance of 0.371 ohm, through which the current of a 

 Grenet battery, exposing 4 square inches of zinc surface, was passing, was 

 introduced into the centre of a large wire coil, whose resistance was 0.295 

 ohm, connected with the galvanometer. The deflection produced was 

 20°. The same deflection was observed on making and breaking contact 

 with the battery, the smaller coil remaining within the larger. 



3. A coil of No. 14 copper wire, sixty centimeters in diameter, and con- 

 taining about 40 turns, the resistance of which was 0.85 ohm, was connected 

 with the galvanometer, and placed on the floor. Raising the south side 

 six inches, caused a deflection of 4°. Placing the coil with its plane ver- 

 tical, a movement of two centimeters to the right or left, caused a deflec- 

 tion of 30, and of twenty centimeters, of 10°. A rotation of 90° gave a 

 deflection of 12° and one of 180°, of 24°. These deflections were of 

 course due to currents generated by the earth's magnetism. 



4. Thermo-currents. — Two pieces of No. 22 wire fifteen centimeters 

 long, were taken, the one of copper, the other of iron wire, and united at one 

 end by silver solder. On connecting the other ends to the galvanometer, 

 the heat of the hand caused a deflection of the needle of 20°. 



5. A thermo-pile of 25 pairs, each of bismuth and antimony, was con- 

 nected to the instrument. The heat from the hand placed at five centi- 

 meters distance caused a deflection of 3°. 



