326 ELEMENTARY LESSONS ON [CHAP. vi. 



was made to rotate very rapidly and uniformly. Such a ring 

 in rotating cuts the lines of force of the earth's magnetism. 



The northern half of the ring, 

 in moving from west toward 

 east, will have (see Rule Art. 

 395) an upward current in- 

 duced in it, while the southern 

 _ . half, in crossing from east to- 

 ward west, will have a down- 

 ward current induced in it. 

 Hence the rotating ring will, 

 as it spins, act as its own 

 galvanometer if a small mag- 

 net be hung at its middle ; 

 p. the magnetic effect due to the 



rotating coil being propor- 

 tional directly to the horizontal component of the earth's mag-' 

 netism, to the velocity of rotation, and to the number of turns 

 of wire in the coil, and inversely proportional to the resistance 

 of the wire of the coils. Hence, all the other data being 

 known, the resistance can be calculated and measured as a 

 velocity. Our present ohm was constructed by comparison with 

 this rotating coil ; but there being some doubt as to whether 

 the existing ohms really represent IO 9 centims. per second, a 

 re-determination of the ohm is at present (1881) in progress, 

 under the auspices of the British Association. 



While these sheets are passing through the press, a determination of the 

 value of the ohm by Lord Rayleigh, shows that the standard coils called 

 " one ohm," are in reality only 0*9895 of the theoretical io 9 absolute units. 



NOTE ON THE RATIO OF THE ELECTROSTATIC 

 TO THE ELECTROMAGNETIC UNITS. 



365. If the student will compare the Table of Dimensions of 

 Electrostatic Units of Art. 258 with that of the Dimensions of 

 Electromagnetic Units of Art. 324, he will observe that the dimen- 

 sions assigned to similar units are different in the two systems. 

 Thus, the dimensions of " Quantity" in electrostatic measure are 



M* L^T , and in electromagnetic measure are M^ L 2 ' 

 Dividing the former by the latter we get LT" 1 , a quantity 



