for  the  Determination  of  Resonance-Carves.  677 
The  accurate  determination  of  the  logarithmic  decrement  in 
open  oscillation  circuits  such  as  these  is  a  matter  of  consider- 
able practical  importance,  as  most  of  the  damping  is  due  to 
the  radiation  of  energy  from  the  wire,  and  not  to  the  resistance 
as  is  the  case  with  closed  or  non-radiative  circuits ;  and  as 
the  amount  of  this  radiant  energy  is  not  easily  estimated 
mathematically,  the  value  of  this  important  quantity  must 
rest  on  experimental  evidence  alone. 
The  objection  may  be  raised  that  the  type  of  ammeter 
described  greatly  increases  the  decrement  of  the  cymometer, 
as  its  decrement  is  almost  equal  to  that  of  the  cymometer 
per  se.  It  must  be  remembered,  however,  that  the  part 
of  the  instrument  in  the  cymometer  circuit  is  a,  short  piece 
of  very  fine  wire  whose  decrement  can  be  easily  and  accu- 
rately calculated,  and  secondly  that  the  decrement  of  the 
cymometer  itself  does  not  matter,  the  primary  circuit  being 
generally  the  one  whose  decrement  is  required. 
At  first  sight  it  would  seem  that  a  very  obvious  variation 
of  the  method  could  be  employed  which  would  not  suffer 
from  this  defect. 
Suppose  that,  instead  of  cutting  the  cymometer-bar,  a  few 
turns  of  well-insulated  wire  are  wound  round  a  section  of  the 
inductance-coil  of  the  cymometer,  and  the  ends  of  the  coil 
connected  to  the  fine  wire  of  the  ammeter  described  above, 
then  currents  will  be  induced  in  this  circuit  proportional  to 
the  currents  in  the  cymometer.  This  method,  however,  has 
more  serious  drawbacks  than  the  other,  as  : — 
(1)  The  damping  of  the  cymometer  will  be  just  as  great  if 
this  method  is  employed,  as  the  same  amount  of  energy  has 
to  be  supplied  to  heat  the  ammeter  wire  to  a  definite 
temperature. 
(2)  The  actual  current  in  the  cymometer  is  not  measured 
but  only  a  current  bearing  some  unknown  ratio  to  it. 
(3)  The  scale  of  the  cymometer  is  slightly  altered  when 
the  ammeter  is  in  place,  as  the  arrangement  is  of  the  nature 
of  a  transformer  with  a  closed  secondary  circuit,  part  of  the 
cymometer  inductance  spiral  forming  the  primary;  this  will 
of  course  annul  a  portion  of  the  inductance  and  so  alter  the 
scale. 
The  extent  of  this  alteration  of  scale  may  be  determined 
by  measurements  made  at  the  point  of  resonance  with  the 
Neon  tube,  first  with  the  secondary  open,  and  second  with  the 
secondary  closed  through  the  ammeter  wire. 
Several  curves  have  been  obtained  using  the  ammeter  in 
this  manner,  but  as  the  method  shows  no  points  of  advantage 
