294 Danger in Apparatus for Distribution of Electricity . 



machine, we have 



KR.2? + x — 27niKA cos Zirnt, 



x = ; T ^T^ — 79 — r ( ~" 27raKR sin 27rn£ + cos 2irnt \ , 

 (KR27m) 2 +l l " 



mean square or x — ,_. .mean square of A. 



Let us now consider the actual values likely to occur in 

 practice. Let the condenser E be a secondary generator ; let 

 the resistance R be that of some person touching some part of 

 the secondary circuit, and also making contact to earth with 

 some other part of the body ; n may be anything from 100 to 250, 

 say 150 ; K will depend on the construction of the secondary 

 generator — it may be as high as 0*3 microfarad or even more, 

 but there would be no difficulty even in large instruments in 

 keeping it down to one hundredth of this or less. The mean 

 square of A will depend on the circumstances of other parts 

 of the circuit ; it might very easily be as great, or very nearly 

 as great, as the mean difference of potential between the ter- 

 minals of the machine if the primary circuit were to earth 

 at C. Suppose, however, that the circuit B D C is symme- 

 trical, that E is at one end, and that another person of the 

 same resistance as the person at E is touching the secondary 

 circuit of the secondary generator F at the other end of the 

 circuit. In that case, if 2400 be difference of potential of the 

 machine, mean square of A will be 1200 ; in which case we 

 have, taking R as 2000 ohms, 



. 2ttx150x0-3x10- 6 tOAA 



mean square of x= , =- x 1200 



4 v/(2tt x 150 x 0-3 x 10- 6 x 2000) 2 + 1 



= about 0*3 ampere. 



Experiments are still wanting to show what current may be 

 considered as certain to kill a man, but it is very doubtful 

 whether any man could stand 0*3 ampere for a sensible length 

 of time. It is probable that if the two persons both took firm 

 hold of the secondary conductors of E and F, both would 

 be killed. If the person at F be replaced by an accidental 

 dead earth on the secondary circuit of F, the person at E 

 w T ould experience a greater current than 0*3 ampere. 



It follows from the preceding consideration that secondary 

 generators of large electrostatic capacity are essentially dan- 

 gerous, even though the insulation of the primary circuit and 

 of the primary coils from the secondary coils is perfect. The 

 moral is — for the constructor, Take care that the secondarv 

 generators have not a large electrostatic capacitv, say not 





