The  Theory  of  Phasemeters.  81 
purified  the  sample  used,  and  one  definitely  states  that  he 
used  a  "  commercial  preparation"" ;  this  would,  without  doubt, 
contain  both  moisture  and  hydrochloric  acid,  namely,  a  con- 
centrated solution  of  the  latter,  the  amount  o£  which  would 
increase  with  time  and  other  conditions. 
Personally  I  experienced  such  difficulty  in  purifying  a 
sample  of  ethylene  dibromide  (though  of  correct  melting-  and 
boiling-point)  so  as  to  be  fit  for  a  determination  of  the 
dielectric  constant,  that  the  attempt  was  for  a  time  abandoned. 
Turner*  gives  a  value  4*865  for  this  substance,  but  makes  no 
allusion  to  such  difficulty. 
It  is  hoped  to  proceed  with  further  determinations  with 
this  form  of  apparatus  and  method  of  working  ;  meanwhile 
it  is  thought  that  they  possess  sufficient  novelty  and  give 
sufficiently  accurate  results  to  merit  description. 
Y.   The  Theory  of  Phasemeters. 
By  W.  E.  Sumpner,  D.Sc.f 
PHASEMETERS  are  instruments  of  the  dynamometer 
type  for  indicating  the  phase  relations  of  the  currents 
and  potentials  in  alternating-current  circuits.  With  few 
exceptions  they  are  made  for  use  on  multiphase  circuits. 
Single  phase  instruments  have  been  constructed,  but  they 
are  not  very  satisfactory,  as  their  calibration  alters  with  both 
frequency  and  wave-form. 
Such  instruments  consist  essentially  of  two  sets  of  coils, 
of  which  one  set  is  fixed  and  the  other  forms  a  single  moving- 
system  which  is  not  provided  with  any  form  of  control. 
The  currents  in  one  set  of  coils  are  determined  by  the  voltages 
of  the  main  circuits  ;  while  those  in  the  other  set  are  pro- 
duced by  the  circuit  currents.  The  total  number  of  coils 
used  in  the  two  systems  must  be  at  least  three,  and  as  a 
rule  actual  instruments  only  contain  this  minimum  number 
of  coils  :  the  fixed  system  usually  consists  of  a  single  coil 
conveying  one  of  the  line  currents,  while  the  moving  system 
consists  of  two  independent  but  relatively  fixed  coils,  traversed 
by  currents  produced,  through  suitable  non-inductive  re- 
sistances, by  two  of  the  voltages  of  the  multiphase  circuit. 
There  is  one  instrument  made  commercially  for  three-phase 
circuits  which  is  of  a  more  elaborate  construction,  and 
contains  three  fixed  coils  for  the  currents,  and  a  moving- 
system — also   of   three    coils — for   the   voltages.      Such   an 
*  Loc.  cit.  supra. 
f  Communicated  by  the  Physical  Society :  read  October  27, 1905. 
Phil.  Mag.  S.  6.  Vol.  11.  No.  61.  Jan.  1906.  G 
