ELECTRICAL MEASUREMENTS. 287 



164. Measurement of magnetic flux. Consider an iron rod 

 through which a certain amount of magnetic flux passes, due, for 

 example, to the magnetizing action of a winding of wire through 

 which a current is flowing. A reversal of this magnetizing cur- 

 rent produces a sudden reversal of the magnetic flux 4> through 

 the rod, so that the total change of flux (from -f 3> to <) is 

 equal to 2<I>. An auxiliary coil having Z turns of wire is 

 placed upon the iron rod and connected to a ballistic galvanom- 

 eter, and the throw d of the ballistic galvanometer is observed at 

 the instant of reversal of the magnetizing current. Then we 

 have 2<J>Z= k'd, inasmuch as the product of the change of flux 

 2$ and the number of turns of wire in the coil gives the impulse 

 value of the electromotive force, and this is equal to k' d. The 

 reduction factor k' of the ballistic galvanometer being known,* 

 the value of 4> can be easily calculated. 



MEASUREMENT OF MAGNETIC FIELDS. 



165. Gauss's method for measuring the horizontal component H' 

 of the earth's magnetic field, and for measuring the magnetic 

 moment of a magnet. This method involves two independent 

 sets of observations, the first set being made with a certain 

 arrangement of apparatus and the second set being made with a 

 different arrangement of apparatus, as follows : 



First arrangement. A large magnet is suspended horizontally 

 at the place where H' is to be determined, set vibrating about 

 the vertical axis of suspension and the time t of one complete 

 vibration is determined by observation. Then from equation 

 (23) we have 



^^ = mlH' (i) 



The moment of inertia K of the magnet/ is to be determined 

 from the measured dimensions and weight/(in grams) of the bar. 



*A method for determining the value of k' is described on page 18, Vol. II, 

 Practical Physics, Franklin, Crawford and MacNutt. 



