536 



I may add here that with 25 ampère the field-strength increased 

 to 40000 gauss. I intend to arm this electro-magnet with pole-pieces 

 made of fei-i'ocobalt, the new alloy of Weiss and hope to obtain 

 with them a maximum of 43000 gauss. 



ill. Determination of the active Jieldstrength. 



As active field-strength we indicated the intensity of a field 

 homogeneous over the whole length of the string, which causes the 

 same detiection as the not homogeneous field, tiie tension and the 

 material of tlie siring as well as the current passing through it, 

 being the same. 



EiNTHOVEN has already given a method to measure the active 

 intensity of the field. His method depends on the measurement of 

 the electro-magnetical damping of the movement of the string. 



Here follow two more methods which lead both to about the 

 same result. 

 First Method. 



If a current / passes through the string, it is subjected to a lateral 

 pressure HI1 which causes the string to sway, and whereby a certain 

 quantity of potential energy is stored. When the circuit is broken the 

 string generally resumes its former position after a few damped 

 oscillations. With every oscillation potential energy is changed into 

 kinetical energy, and vice versa. 



At the moment the string passes for the first time its final position 

 of equilibrium, the total originally stored 

 potential energy has disappeared, and nothing 

 but kinetical energy remains. If the oscillations 

 had been undamped, we might equal these 

 two forms of energy. On account of the damp- 

 ing the value for the kinetical energy must 

 first be corrected. We have then got an 

 equation from which H can be resolved, and 

 _-^ which gives an expression for the actual mean 

 intensity of the magnetic field. 



In our considerations we assume the field 

 to be homogeneous and we neglect the elasti- 

 city and the vveight of the string itself with 

 regard to its strain. Under these circumstances 

 the string takes, as we saw, a parabolic shape. 

 Be BQOD the defiected string, its centre 

 tig- ^- being 0. With regard to a system of coordi- 



