458 Messrs. Guthrie and Boys on 



may be taken as directly proportional to their joint thickness 

 the mean distance remaining the same. There is probably a 

 misreading with the four disks. 



VI. Effect of Nature of Metal. — Sheets of copper, zinc, tin, 

 brass, and lead having been rolled between the same rollers, 

 were found to have the thickness as under : — 



millim. 

 Copper .... 0-89 



Zinc 0-87 



Tin 0-71 



Brass 0'93 



Lead 0-76 



Disks 2*875 inches in diameter were cut out of the sheets, 

 and fastened in the usual way by a drop of gum to the under 

 surface of the ebonite disk. The results are given in Table VI. 



Table VI. 



Metal. 



A. 



T. 



A'=AT. 



t ' 



r. 



A'xr 

 t 



Copper... 



78-5 



218 



17113 



1923 



•Ill) 



229 



Brass . . . 



28-8 



230 



662-4 



712 



•423 



301 



Zinc 



314 



23-0 



722-2 



830 



•326 



271 



Tin ... 



13-0 



233 



302-9 



427 



•669 



286 



Lead . . . 



8-4 



230 



193-2 



254 



1-109 



281 



The column headed — - is obtained by dividing the numbers 



in the previous column by the thickness of metal used. The 

 numbers in the column r, give the resistance in ohms of 100 

 inches of wire drawn from the same metal that was rolled into 

 sheets, and "79 millim. in diam. These two columns multiplied 

 together give the last column, which should be a constant if 

 the deflexion is proportional to the conductivity. Now these 

 numbers range from 229 to 301, which seems rather a wide 

 divergence ; but in this case there are many considerations to 

 be taken into account, in each of which errors may easily 

 creep in, especially in the assumption that the specific conduc- 

 tivity of the same metal is the same whether it be rolled or 

 drawn. The approximation to equality, obtained in the num- 

 bers in the last column, is all that could be expected. 



It appears from the above experiments in I., II. , III., IV., 

 V., VI., that the torsion varies directly with the moment of 

 the current. 



VII. Examination of Liquids. — The electro-magnet seen in 

 vertical section in PI. XIII. fig. 2, one third true size, was made 

 to screw onto a vertical axis, so as to revolve in a horizontal plane. 



