166 HENKY A. EOWLAND 



necessary to describe here only those portions which affect the accuracy 

 of the present experiment. 



The coils were of the form described above in the theoretical portion, 

 and were wound on a brass cylinder about 8-2 cm. long and 11-6 cm. 

 diameter in two deep grooves about 3- cm. deep and 2-5 cm. wide. The 

 opening in the centre for the needle was about 5-5 cm. diameter and 

 the cylinder was split by a saw-cut so as to diminish the damping 

 effect. This coil was mounted on a brass column rising from a gradu- 

 ated circle by which the azimuth of the coil could be determined by 

 two verniers reading to 30". Through the opening in the coil beneath 

 the needle passed a brass bar 95 cm. long and 2 cm. broad, carrying a 

 small telescope at one end. In the present experiment, this bar was 

 merely used in the comparison of the constant of the instrument with 

 that of another instrument. For this purpose the instrument is used 

 as a sine galvanometer by which a great range can be secured, and it 

 could be compared with a coil having a constant twenty-three times 

 less and which was used with telescope and scale. 



The coils contained about five pounds of No. 22 silk-covered copper 

 wire in 1790- turns. 



Two needles were used in this galvanometer, each constructed so that 

 its magnetic axis should be invariable; this was accomplished by affixing 

 two thin laminae of glass-hard steel, to the two sides of a square piece 

 of wood, with their planes vertical. This made a sort of compound 

 magnet very strong for its length, and with a constant magnetic axis. 

 The first needle had a nearly rectangular mirror 2-4 by 1-8 cm. on 

 the sides and -22 cm. thick. The other needle had a circular mirror 

 2-05 cm. diameter and about 1 mm. thick. The needle of the first was 

 1-27 cm. and of the second 1-20 cm. long, and the pieces of wood were 

 about -45 cm. and -6 cm. square respectively. The moment of inertia 

 of both was much increased by two small brass weights attached to 

 wires in extension of the magnetic axis, thus extending the needles to 

 a length of 4-9 cm. and 4-2 cm. respectively. The total weights were 

 5-1 and 5-6 grams and the times of vibration about 7-8 and 11-5 

 seconds. They were suspended by three single fibres of silk about 43 

 cm. long. 



In front of the needle was a piece of plane-parallel glass. This and 

 the mirrors were made by Steinheil of Munich, and were most perfect 

 in every way. 



In the winding of the coils every care was taken, seeing that a small 

 error in so small a coil would produce great relative error. And for 



