34     Mr.  J.  E.  H.  Gordon  on  a  new  Anemometer  for  Indicating 
16  points  to  the  compass;  the  instrument  can,  however,  be 
made  to  give  32  or  64  if  required.  On  this  axis  are  two  con- 
tact-breakers ;  one  makes  a  contact  for  each  complete  revolution 
in  one  direction,  the  other  for  each  complete  revolution  in  the 
other  direction. 
The  contact-breakers  consist  of  two  ebonite  disks.  Fig.  2  is 
a  front  view  of  one  of  the  disks.  In  the  surface  of  it  are  turned 
two  grooves — one  concentric  with  the  axis  of  the  disk,  the  other 
excentric  and  of  larger  radius  than  the  first ;  the  two  grooves 
run  into  each  other  for  rather  more  than  half  their  circurnfe* 
rence.  In  the  grooves  are  two  slides  or  valves;  these  move  on 
a  pin  at  one  end,  the  other  end  has  a  piece  of  wire  fastened  to 
it ;  this  wire  passes  through  a  little  curved  slot  at  the  bottom  of 
the  groove  (the  radius  of  curvature  equals  the  length  of  the 
valve) ;  the  wire  projects  right  through  the  slot,  and  is  pressed 
on  by  a  spring  fixed  on  the  back  of  the  disk.  In  one  of  the 
disks  the  valves  are  pressed  by  the  springs  into  the  positions 
shown  in  fig.  2 ;  in  the  other  disk  the  springs  press  them  in  the 
opposite  direction. 
Near  the  disks  are  slips  of  thin  brass  fixed  on  little  upright  pil- 
lars and  insulated  from  one  another.  The  ends  of  the  brass  slips 
are  fixed  at  right  angles  to  them,  and  dip  into  the  grooves.  The 
ends  are  made  circular,  and  can  revolve  so  as  to  lessen  the  friction 
against  the  sides  of  the  grooves.  When  the  disk  a  revolves  in  the 
direction  of  the  arrow,  the  point  of  its  brass  slip  is  caused  by  the 
valve  to  travel  in  the  concentric  inner  groove  and  no  effect  is 
produced.  When,  however,  the  disk  turns  in  the  direction  op- 
posed to  the  arrow,  the  point  travels  in  the  excentric  outer 
groove,  the  brass  slip  is  depressed,  a  platinum  button  on  it 
presses  on  a  corresponding  platinum  point  on  a  spring  placed 
just  below  the  brass  slip,  and  a  current  passes  between  the  points. 
The  valves  of  the  two  disks  are  so  arranged  that  the  direction  of 
motion  which  makes  contact  by  one  disk  does  not  make  contact 
by  the  other,  and  vice  versa. 
The  weight  at  the  left-hand  side  of  fig.  1  is  most  important. 
It  revolves  with  the  disks,  and  prevents  the  vane  ever  stopping 
with  either  point  in  contact.  If  it  were  not  for  this,  the  points 
might  remain  in  contact,  and  a  battery  which  ought  to  last  six 
months  would  be  worked  out  in  an  hour  or  two.  It  also  pre- 
vents the  vane  making  a  contact  and  then  going  back  without 
passing  a  whole  point. 
In  order  that  the  weight  may  work,  the  crown  wheel  is  not 
rigidly  keyed  on  to  the  axis,  but  has  two  stops  fixed  on  it  a 
short  distance  apart;  a  pin  fixed  to  the  axis  projects  between 
the  two  stops.  The  crown  wheel  has  a  play  which  is  made  equal 
to  half  a  revolution  of  the  disks. 
