Cymometer  for  Determination  of  Resonance-  Carves.       665 
the  quantity  of  electricity  which  has  to  be  carried  is  small 
and  the  electronic  emission  from  even  a  4-volt  1-ampere 
carbon  filament  is  amply  sufficient  to  carry  the  negative 
component  of  the  feeble  oscillations  used  across  the  vacuous 
space. 
It  should  be  noted  that  such  oscillation-valves  as  are  here 
described  have  quite  a  different  range  of  use  from  other 
rectifying  arrangements  such  as  the  Cooper-Hewitt  mercury- 
vapour  tube,  and  the  electrolytic  aluminium-carbon  valve  of 
Nodon  and  others. 
The  electrolytic  valves  produce  no  rectifying  effects  with 
high-frequency  alternating  currents,  because  the  time  element 
enters  into  the  formation  of  the  aluminic  hydroxide  film  on 
which  their  action  depends.  On  the  other  hand,  the  mercury- 
vapour  tubes  which  have  been  proposed  for  use  with  high- 
tension  alternating  currents  will  not  operate  below  a  certain 
minimum  potential-difference  between  the  electrodes.  The 
vacuum-valve  as  here  described,  however,  will  pass  current 
unilaterally  with  a  fraction  of  a  volt  difference  of  potential 
between  the  incandescent  and  the  cold  electrode,  and  there 
is  no  minimum  potential  difference  below  which  they  will 
not  act  ;  hence  their  use  is  conditioned  solely  by  the  sensi- 
tiveness of  the  galvanometer  employed  with  them. 
By  its  simplicity  and  ease  of  use  the  carbon-filament 
vacuum-valve  recommends  itself  as  a  useful  addition  to  our 
resources  for  experimental  work  in  connexion  with  electric 
oscillations  and  electric-wave  telegraphy. 
LX.    On  the  Use  of  the  Cymometer  for  the  Determination  of 
Resonance- Curves.     By  Gr.  JB.  Dyke,  B.Sc* 
DR.  FLEMING  has  shown  in  his  recent  Cantor  Lectures 
before  the  Society  of  Arts  t,  that  by  the  introduction 
of  a  hot-wire  ammeter  into  the  circuit  of  his  direct-reading 
cymometer,  the  effective  or  root-mean-square  value  of  the 
oscillation  current  set  up  in  the  cymometer  circuit  can  be 
measured.  This  instrument  was  originally  designed  for  the 
determination  of  the  wave-lengths  used  in  wireless  telegraphy 
by  the  direct  inspection  of  a  scale,  and  also  for  the  measure- 
ment of  capacities  and  inductances;  but  it  has  been  found 
that  a  small  addition  renders  the  instrument  also  available 
for  the  determination  of  resonance-curves,  and  therefore  of 
*  Communicated  by  the  Physical  Society  :  read  March  23,  1906. 
t  Cantor  Lectures,  1905.     Dr.  J.  A.  Fleming  on  "  The  Measurement 
of  High  Frequency  Currents  and  Electric  Waves." 
Phil.  Mag.  S.  6.  Vol.  11.  No.  65.  May  1906.         2  X 
