290 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



known in equation (i). The quantities d t d l , <f> and <f> l in 

 equation (vii) are observed, leaving only ?/ and H' unknown 

 in (vii). Equations (i) and (vii) then enable the calculation of 

 both i/ and H'. 



If it is desired to determine the strength of the poles of the 

 large magnet, the quantity / may be approximately measured, 

 and m calculated. 



This method * for determining ml and H was devised by 

 Gauss. 



166. Measurement of ^magnetic field intensity by means of the 

 tangent galvanometer. When the value of the horizontal com- 

 ponent of the earth's magnetic field H' is known, the tangent 

 galvanometer may be used to measure the value of the current 

 in amperes or abamperes, as explained in Art. 57. If a known 

 current (measured by a copper coulombmeter, for example) is 

 sent through a tangent galvanometer and the deflection < ob- 

 served, then the value of H' may be calculated, the number of 

 turns of wire Z and the mean radius r of the coil being 

 known. 



167. Measurement of magnetic field intensity by means of the 

 bismuth inductometer. The bismuth inductometer is a small 

 resistance coil made of fine bismuth wire. Its resistance varies 

 with the intensity of the magnetic field in which it is placed. 

 The relation between resistance and field intensity being once for 

 all determined, the intensity of any field may be found by meas- 

 uring the resistance of the inductometer when it is placed in the 

 field. 



168. Kohlrausch's method for the simultaneous absolute measure- 

 ment of the horizontal component of the earth's magnetic field and 

 of current. The coil of a tangent galvanometer is suspended 

 so as to enable the measurement of the torque T with which 

 the earth's horizontal field H' acts upon it. This torque is given 



* For fuller discussion of Gauss' s method, see A. Gray, Absolute Measurements 

 in Electricity and Magnetism, Vol. II, page 69. 



