308 



Mr. W. Hibbert on a 



But the growth of the modern idea of a magnetic circuit 

 suggested the possibility of achieving my purpose in a way 

 that would be largely independent of the peculiarities of 

 different brands of steel, as well as of the various physical 

 conditions caused by differences in tempering. 



I therefore provided a short straight bar-magnet with a 

 couple of arched pole-pieces, of such length and sectional area 

 that there was left between them a narrow air-gap of very 

 small magnetic " resistance." A very flat coil of wire thrust 

 in or out of this gap gave electromagnetic impulses whose 

 value was fairly constant. Experience with these simple 

 instruments led me to believe that a more rigorous application 

 of the principle would give a truly permanent magnetic field. 

 How far this has been realized will appear from the con- 

 siderations which I now have the honour to submit to the 

 Society. 



The first design that at all approximated to a closed mag- 

 netic circuit consisted of a cylindrical steel rod with hemi- 

 spherical pole-pieces. For reasons into which I need not go, 

 this has been superseded by the following modification. 



Fig. 1. 



Fig 1 is a general view of the instrument. Fig. 2 is a 



