INTRODUCTORY. 3 



a sphere of radius r centimetres, the number of lines of force 

 crossing each square centimetre of surface on the sphere is 



m 



that is, the force at any point on the surface of the sphere is 

 numerically equal to the number of lines cutting the sphere per 

 square centimetre of surface. 



It is to be noticed that these lines of force cut the surface of 

 the sphere normally. By this convention, therefore, we can say 

 that the strength of a magnetic field at any point is measured by 

 the number of lines of force passing normally through a square 

 centimetre with the given point at its centre. 



The number of lines of force per square centimetre passing 

 normally through a surface is called the Induction through the 

 surface. 



The total number of lines of force passing through a surface is 

 called the Total Flux through the surface. 



Uniform Field If the lines of force in a given region 

 arc everywhere parallel to the same direction, and if the induction at 

 every point is the same, the field of force is said to be uniform. 



4, Magnetic Field due to an Electric Current. 

 When an electric current flows along a conducting wire a 

 magnetic field is produced, the lines of force of which are closed 

 curves, some within the substance of the wire and the others 

 threading through the circuit. 



Unit Current. The unit of current is defined to be that 

 current which, flowing in a conducting wire one centimetre long 

 bent into the arc of a circle of one centimetre radius, acts on unit 

 magnetic pole placed at the centre with a force equal to one dyne. 



The practical unit of current the Ampere is one-tenth of 

 the C.G-.S. unit of current thus defined. 



Strength of Field inside a Solenoid. The 

 magnetic field inside a helix of many turns carrying a current 

 of i C.G.S. units is practically uniform at points at a moderate 

 distance from the ends, and its strength is equal to ^win, where n 

 is the number of turns per centimetre measured along the axis of 

 the helix. If the current is i amperes, the strength of the magnetic 

 field is given by 



F=^ ....... (3) 



A helix of many turns is sometimes called a Solenoid. 



