670         Mr.  G.  B.  Dyke  on  the  Use  of  the  Cymometer 
This  equation  gives  us  the  sum  of  the  decrements  in  the 
two  coupled  circuits.  In  order  to  obtain  the  values  o£  8j_ 
and  B2  separately  we  require  some  other  relation  between 
them.  This  relation  has  been  given  by  Drude.  He  shows 
that  the  resonance  current  in  the  secondary  circuit  can  be 
calculated  from  a  formula  equivalent  to 
T2        tt  ?Cl02  TT^P 
<Jr—  \1 
8   y2ft+y 
where  V1=maximum  value  of  primary  condenser  potential- 
difference. 
Ci  and  C2  =  capacities    in    primary    and    secondary    circuits 
respectively. 
k  =  coefficient  of  coupling  of  the  two  circuits 
M 
\ZL-Jj2  ' 
where  M  =  mutual  inductance  between  the  circuits  ; 
Lx  and  L2  =  self-inductances  of  the  primary  and  secondary 
circuits  respectively. 
Passing  now  to  the  delineation  of  the  resonance  curve, 
we  proceed  as  follows  : — Insert  the  hot-wire  ammeter  into 
the  cymometer  circuit  and  place  the  cymometer  parallel  to 
some  straight  portion  of  the  primary- circuit  and  at  such  a 
distance  from  it  that  when  the  cymometer  is  adjusted  to 
resonance  the  current  is  not  too  large  to  be  measured  on  the 
galvanometer.  Then  move  the  cymometer  handle  slowly 
from  one  end  of  the  scale  until  a  readable  deflexion  appears 
on  the  galvanometer.  From  this  point  move  the  handle 
in  small  steps,  noting  at  each  step  the  current  J  in  the 
cymometer  as  given  by  the  calibration  curve  of  the  galvano- 
meter, and  the  frequency  n2  as  read  on  the  cymometer-scale. 
Proceed  in  this  way  until  the  maximum  deflexion  is  passed 
and  the  current  has  again  fallen  to  a  small  value.  Plot  the 
results  thus  obtained  as  a  curve  having  ordinates  proportional 
to  J,  and  abscissae  proportional  to  n2.  Take  off  from  this 
curve  the  maximum  value  of  J.  This  will  be  the  resonance 
current  Jr;  and  the  corresponding  frequency  will  be  the 
resonance  frequency,  that  is,  the  frequency  n:  of  the  primary 
circuit.  Now  repeat  the  observations  inserting  the  auxiliary 
resistance  into  the  cymometer  circuit  in  addition  to  the 
ammeter,  taking  care  not  to  alter  the  relative  positions  of 
the  circuits  in  so  doing.  Plot  the  results  as  before  and 
obtain  the  value  of  the  resonance  current  Jr'.  The  resonance 
frequency    should    of    course    remain    unaltered.      The    two 
