PEESIDENTIAL ADDRESS — SECTION A. 29 



As the measurement of very small forces enters at present 

 largely into all scientific work, and is very likely in the future 

 to enter still more largely, the importance of this, one of the 

 many of Professor Boys's discoveries, can scarcely be over- 

 estimated. 



In this connection, mention should also be made of a most 

 difficult and valuable piece of researcli — the decermination of 

 the elastic constants of quartz fibres, which has lately been 

 performed by Professor Threlfall, my predecessor in the pre- 

 sidency of this section. Considering the many uses that in 

 future scientific work these fibres can be put to, these results 

 of Professor Threlfall's labours are of extreme value. 



Within the last few years an enormous extension of our 

 knowledge about magnetic induction has taken place. This 

 has undoubtedly been called forth by the necessity for the 

 knowledge of correct principles on which dynamos and traiis- 

 formers could be constructed. 



It is a well-known fact that when a voltaic current passes 

 along a w^ire it establishes a magnetic field in the medium sur- 

 rounding the wire, and that the direction and magnitude of the 

 magnetic force at any point of the field can be very simply 

 stated by aid of Faraday's conception of lines of force, the lines 

 being parallel to the direction, while if the medium be a vacuum 

 or air the number of them that passes through an area of one 

 square centimetre represents the force perpendicular to that 

 area. 



It was discovered by Faraday that if a second conductor 

 forming a loop be suddenly brought into the field {i momentary 

 current would be produced in it, and he proved that the whole 

 quantity of electricity that flowed round, multiplied by the 

 resistance of the loop, was numerically equal to the number of 

 lines of force piercing the loop that had been added by the 

 motion. If a ballistic galvanometer be included in the circuit 

 of this loop its needle will be deflected, and it is well known 

 that the sine of half the angle of deflection is proportional to 

 the total quantity of electricity that has formed the momentary 

 current. Thus we have a method of measuring the increase or 

 decrease of the number of lines of magnetic force piercing the 

 coil due to the motion. 



Let us now suppose that we have a ring of any substance 

 wound uniformly with an insulated wire in the circuit of 

 which we have a battery, a commutator for reversing the direc- 

 tion of the current at will, a tangent galvanometer for measuring 

 the current, and a set of resistances with which we can vary 

 its intensity. Call this the primary circuit, and let there be 

 n turns of the wire per centimetre of length of the ring. 



Let us also at any point of the ring have for simplicity a 

 single turn (in practice a number of turns would be necessary) 



