508 



PROFESSOR W. E. AYRTON, MR. T. MATHER AND MR. F. E. SMITH: 



Here j and / represent the lower and upper ends respectively of the left-hand 

 suspended coil, whilst CBD and GJH indicate the lower and upper helices on the 

 left-hand fixed cylinder. B and J are supposed to be on the mid-planes of the 

 respective helices. The right-hand suspended and fixed coils are similarly represented 

 by a, and a,' and C'B'D', G'J'H'. 



Fig. 21. Vertical section through coils of current weigher. 



If only one pair of coils be used for making a determination, the change of apparent 

 mass due to a reversal of current in the fixed coils enables the current strength to be 

 calculated directly. Using both pairs, however, introduces cross actions between 

 them, and the forces due to these must be calculated or eliminated. 



To distinguish the forces between the coils on one pair of cylinders from those 

 between the suspended coil of one pair and the fixed coils of the other pair, we have 

 called them " direct " and " secondary " forces respectively. For example, the forces 

 between a lf a/ and CD, GH are called " direct forces," whilst the vertical component of 

 the force exerted on a,, a/ by CT)' and G'H' is called a " secondary " force. For 

 shortness, these are designated by D and S. 



A little consideration will show that when the current in a 1( a/ is in the same sense 

 as that in 2 , a/, and both sets of coils are in use, assisting, the electromagnetic force 

 operative is the sum of the direct and secondary forces (D + S), whilst if the currents 

 in the suspended coils are opposite in sense, the resultant force is (D S).* Two sets 

 of observations are therefore necessary to eliminate the secondary forces, t 



Horizontal components of the cross forces will exist, as well as forces due to the 

 action of the suspended coils on each other tending to push them apart or pull them 

 together. These forces, however, are so small compared with the mass of the 

 suspended cylinders that no appreciable displacement is produced. Careful observa- 

 tion by a telescope, made with a view to detecting side displacement, led to a 

 negative result. 



Considering one set of coils, say the left-hand ones in fig. 21, the value of (M 2 Mj) 



* In each case the directions of the currents in the two pairs of .coils (left and right hand) are made 

 such as will produce torques on the beam in the same sense. 



t Mr. SEARLE has developed an expression by which these secondary forces may lie calculated. 



