REAL GENERATORS 



The ^bridged T' attenuator — There is a useful and interesting variation of 

 the T section, which, though it contains 4 resistances instead of 3, requires 

 only 2 of them to be varied to vary (J. It may be derived as follows. 



Take an ordinary T section [Figure 2.38); take the R^ and split it into 



«.-(- 



-1 



"mr 



Vv^^ 



-AAA- 



26 



^2-"Vj.e2 



R --(r) 



Figure 2. 38 



two resistances {0/(1 — ())]r and {OfiX -f ())]r {Figure 2.39). These add up to 

 (20)1(1 — 0^) as before. Now transform the star between A, B, and C into a 



•> A 



.R(=r) 



Figure 2.39 



delta (Figure 2.40). This gives the 'bridged T\ Notice that two of the four 

 resistances are fixed and equal the characteristic resistance of the section. 

 The other two have values which are elegantly reciprocal, and easy to remem- 

 ber. 



R(=rJ 



Figure 2.40 



Alternating current in circuits containing only resistance — R.M.S. values 



All the expressions so far derived have begun from the assumption of a 

 direct current or direct voltage generator. They are all equally true for 

 alternating current. Since the variable 'time' has not entered into any of 

 the equations (except those for quantity of electricity and for energy in the 

 very first section) they can be regarded as applying either to steady conditions 

 or to instantaneous conditions; i.e. wherever V and / have appeared, they 

 can be replaced by instantaneous values v and /, equal respectively to V sin (nt 



23 



