[    186    ] 
XX.  On  Testing  the  Metal-resistance  of  Telegraph-wires  or 
Cables  influenced  by  Earth-currents.  By  G.  K.  Winter, 
Telegraph  Engineer }  Madras  Railway*. 
[With  a  Plate.] 
THE  fact,  I  believe,  is  sufficiently  well  known  to  all  who  have 
had  any  thing  to  do  with  the  testing  of  telegraph-wires  or 
cables,  that  it  is  almost  impossible  to  have  two  earth-plates  in- 
serted any  distance  apart  without  a  difference  of  tension,  greater 
or  less  according  to  circumstances,  existing  between  them ;  this 
is  due  in  some  cases  to  earth- currents  properly  so  called,  in 
others  to  polarization  of  the  earth-plates  from  the  passage  of 
currents,  in  others  to  a  difference  between  the  earth-plates  them- 
selves or  the  soil  in  which  they  are  imbedded,  but  generally 
to  these  causes  combined.  The  important  influence  of  the 
currents  in  the  wires  due  to  this  difference  of  tension  upon 
the  apparent  resistance  of  the  wire  when  tested  by  the  Wheat- 
stone's  bridge  has  not,  I  fear,  been  hitherto  fully  appreciated. 
In  Sabine's  <  The  Electric  Telegraph/  pp.  292  and  293,  the 
author  deduces  from  KirchhofFs  laws  the  following  equations 
(see  the  annexed  diagram) : — 
E'_  BC-AD 
^"Ap  +  BO  +  BtA  +  B,)' 
p_  AD      E'  A(D  +  R)+B(A  +  R) 
L-  "F  -  E  ~B  : 
(1) 
(2) 
in  which  E  is  the  electromotive  force  of  the  testing  battery,  and 
E'  the  foreign  electromotive  force  in  the  branch  C. 
*  Communicated  by  the  Author. 
