40 INDUCTION SHOCKS 
connecting the secondaries is, as stated previously, pur- 
posely such that the value of R is constant throughout. 
It therefore disappears from the equation and we have 
MI = M'S. The value of M ' is computed from the 
construction of the standard coil according to the formula 
M' = 4 irnNASj in which n equals the number of turns in 
the primary coil per centimeter of length, N the total 
number of turns in the secondary coil, A the area of the 
cross section of the primary, and S the current through 
the primary in electromagnetic units. Since this current 
is measured in amperes, it is necessary in practice to 
call S the intensity of the primary current in amperes 
and divide the expression by 10 to reduce to electro- 
magnetic units. The formula for M' then becomes 
4 irnNAS 4 irnNA . 
The value - is corstant for any given 
10 10 
standard coil, and once determined is substituted for 
M' in the equation MI = M'S. 
To illustrate the process of determining mutual in- 
ductions by this method, suppose the standard coil has 
the following dimensions: 
Number of turns in primary per centimeter. . . 5.4 
Total number of turns in secondary 1865 
Area of cross section of primary 6 sq. cm. 
The value of - - is 75,870, and the equation for 
10 
mutual induction is M = ' Now suppose that 
