666         Mr.  G.  B.  Dyke  on  the  Use  of  the  Cymometer 
the  decrement  of  oscillation-trains,  and  of  oscillatory  spark- 
resistances. 
A  direct -reading  cymometer  was  used  constructed  as 
described  by  Dr.  Fleming  in  a  paper  read  before  this  Society 
on  March  24th,  1905  *.  The  instrument  consists  essentially 
of  a  closed  circuit  containing  a  condenser  and  an  inductance, 
the  distinctive  feature  being  the  fact  that  the  capacity  and 
inductance  are  so  arranged  as  to  be  varied  simultaneously 
and  in  the  same  proportion  by  one  movement  of  a  handle. 
A  portion  of  the  closed  circuit  consists  of  a  straight  copper 
rod,  which  is  placed  in  the  neighbourhood  of,  and  parallel  to, 
the  circuit  on  which  the  measurements  are  being  made. 
Then,  as  Dr.  Fleming  has  shown  in  the  paper  referred  to 
above,  resonance  will  take  place  between  the  two  circuits 
when  the  cymometer  is  so  adjusted  that  its  oscillation 
constant,  that  is  the  square-root  of  the  product  of  the  capacity 
and  inductance,  has  the  same  value  as  that  of  the  circuit 
under  test.  The  position  of  resonance  is  detected  by  the 
illumination  of  a  jSTeon  vacuum-tube  connected  between  the 
inner  and  outer  coatings  of  the  condenser.  The  Neon 
vacuum-tube,  although  an  excellent  detector  of  the  position 
of  resonance,  gives  but  little  indication  of  the  relative  value  of 
the  current  in  any  other  position  of  the  cymometer,  and  is  of 
the  nature  of  an  indicator  rather  than  a  measuring  instrument . 
For  the  determination  of  the  logarithmic  decrement,  it  is 
requisite  to  know  the  relative  values  of  the  current  in  the 
cymometer  for  points  in  the  neighbourhood  of  resonance, 
and  hence  a  quantitative  current-meter  must  be  employed. 
The  instrument  should  fulfil  the  following  conditions  : — 
(i.)  Its  capacity  and  inductance  should  be  negligible 
compared  with  that  of  the  cymometer,  otherwise  the 
disturbance  of  the  scale- readings  would  lead  to 
erroneous  results, 
(ii.)  Its  damping  factor  should  be   small  and    should    be 
capable  of  calculation, 
(iii.)   It  should  be  fairly  rapid  in  its  action. 
The  above  requirements  are  fulfilled  by  an  addition  to  the 
cymometer,  made  recently  by   Dr.  Fleming,  and   shown  in 
his  Cantor  Lectures  referred  to  above. 
The  ammeter  adopted  is  of  the  hot-wire  type  and  is  inserted 
into  a  gap  made  by  cutting  the  bar  of  the  cymometer  ;  the 
current  passing  through  the  heated  wire  being  measured  by 
a  thermojunction  in  contact  with  it. 
*  u  On  the  Application  of  the  Cymometer  to  the  Determination  of  the 
Coefficient  of  Coupling-  of  Oscillation  Transformers/'  by  Dr.  J.  A. 
Fleming.  Proc.  Phys.  Soc.  Lond.  vol.  xix.  p.  603  ;  and  Phil.  Mag-.  June 
1905 
