484 Prof. J. A. Fleming. Conversion of Electric [Jan. 24, 



one half of the alternating current will be cut out. The galvanometer 

 will read the true mean (T.M.) value of the remanent unidirectional 

 current, and the dynamometer will read the root-mean-square (R.M.S.) 

 value. If the conductivity in one direction is not zero, then the 

 galvanometer will read the T.M. value of the difference of the positive 

 and negative currents, but the dynamometer will read the R.M.S. value 

 of their sum.* 



In the last case, the current through the valve may be considered to 

 be a continuous current superimposed upon an alternating current. 



If we call I the maximum value of the nearly sinoidal current in one 

 direction, and I' the maximum in the opposite direction, then we may 

 say that the dynamometer reading (D) expressed in true current value 

 is equal to a (I + 1') where g is the amplitude factor, and also that the 

 galvanometer reading (G) in true current value is equal to g/f (I - 1') 

 where / is the form facto?' of the current.! Hence 



Jr r D / G+ /- 



-^ 



The fraction - -, say /?, expressed as a percentage may be called 



the rectifying power of the valve, for it expresses the percentage which the 

 actual unilateral electric flow or continuous current through the valve 

 is of that continuous current which would flow if the unilateral con- 

 ductivity were perfect. 



Perfect rectifying power, however, does not exist. There is not an 

 infinite resistance to movement of negative electricity from the metal 

 cylinder to the hot filament through the vacuum, although this 

 resistance is immensely greater than that which opposes the movement 

 of negative electricity in the opposite direction. This point was 

 examined, as follows : A very sensitive electrodynamometer was 

 skilfully constructed by my assistant, Mr. G. B. Dyke, the fixed coil 

 having 2000 turns of No. 47 silk-covered copper wire and the movable 

 coil 1000 turns. The suspension of the movable coil was by a fine flat 

 phosphor-bronze wire at top and bottom. The, deflection was observed 

 by a mirror and scale. 



* If i is the instantaneous value of a periodically varying current with maximum 

 value I and periodic time T, then the root -mean-square value (R.M.S. value) of i 



is defined to be I i l dt \ and the true mean value (T.M. value) of i is defined 



f The form factor f &nd amplitude factor g are the names given by the author 

 (see ' Alternating Current Transformer,' J. A. Fleming, vol. 1, p. 585, 3rd edit.) to 

 the ratio of the E.M.S. to the T.M. value of the ordinates of a single valued 

 periodic curve, and to the ratio of the R.M.S. value of the ordinates to the 

 maximum value during the period. 



