in  a  Discharge  between  Parallel  Plates. 
745 
about  *1  millimetre  in  thickness  and  1  millimetre  apart,  and 
were  parallel  to  the  surfaces  of  the  electrodes. 
A  battery  of  known  electromotive  force  was  connected 
through  a  galvanometer  to  the  electrodes  in  the  tube,  and 
the  potential  of  the  wire  was  gradually  raised  until  sparking 
took  place. 
The  following  table  represents  the  results  of  the  experiments, 
V  denotes  the  potential-difference  between  the  electrodes, 
and  V'the  potential-difference  between  the  negative  electrode 
and  the  gauze  when  sparking  occurred. 
Y 
550 
600 
640 
680 
700 
V 
240 
210 
170 
150 
100 
It  thus  appears  that  the  sparking  potential  between  the 
two  electrodes  may  be  lowered  by  150  volts  by  increasing 
the  strength  of  the  field  near  the  negative  electrode. 
The  same  results  were  obtained  by  a  different  arrangement 
for  charging  the  grating.  A  known  electromotive  force  was 
applied  to  the  electrodes,  and  the  grating  was  insulated  and 
connected  to  one  terminal  of  an  electrostatic  voltmeter,  the 
other  terminal  being  connected  to  the  negative  electrode.  A 
small  quantity  of  radium  was  held  outside  the  tube  opposite 
the  space  between  the  grating  and  the  positive  electrode. 
The  grating  thus  acquired  a  positive  charge  slowly,  and  when 
the  potential  as  shown  by  the  voltmeter  attained  a  certain 
value,  sparking  took  place  between  the  electrodes.  The 
greater  the  potential- difference  between  the  electrodes,  the 
less  is  the  charge  acquired  by  the  grating  before  the  spark 
passes.  The  results  obtained  in  this  way  were  practically 
the  same  as  those  given  in  the  above  table. 
Phil.  Mag.  S.  6.  Vol.  11.  No.  $6.  June  1906.  3  C 
