662 
Prof.  Fleming  on  Oscillation  Valves 
as  a  discharge  surface.  All  who  have  experimented  much 
with  Hertzian  oscillators  know  how  the  state  of  the  polish  of 
the  surface  of  the  metal  balls  (generally  brass)  affects  the 
electric  wave-producing  power.  It  can  be  shown  by  the  use 
of  an  oscillation  waive  that  for  quantitative  work  a  discharger 
made  of  carbon  rods,  as  follows,  presents  many  advantages: — 
A  row  of  arc-lamp  carbons  C,  0,  C  (see  fig.  3)  are  fixed  like 
posts  in  a  piece  of  ebonite  and  another  row  of  slightly  conical 
carbon  rods  B,  B  are  inserted  transversely  between  them,  the 
distances  between  the  rods  being   fixed  so   that  very  small 
Fig.  3. 
TO    INDUCTION     COIL 
air-gaps  are  left  between  carbon  and  carbon.  We  thus  con- 
struct a  multiple  spark-gap  of  carbon  surfaces  which  has  small 
damping  and  great  constancy.  By  enclosing  the  rods  in  a 
non-oxidizing  atmosphere  we  can  prevent  the  rods  burning 
away.  Another  advantage  of  the  arrangement  is  the  ease 
with  which  new  surfaces  can  be  brought  into  use. 
We  can  also  investigate  by  the  same  means  whether  the 
use  of  spark-balls  immersed  in  oil  presents  any  advantages. 
Also  the  same  arrangements  may  be  used  to  exhibit  the 
screening  action  of  conductors  for  high  frequency  magnetic 
