graduated  bar,  and  consequently  exactly  midway
between  the  two  fixed  primary  coils,  no  sounds  are
heard  in  the  telephone,  but  when  moved  to  one
side  or  the  other  the  sounds  are  heard.  Switches
are  so  arranged  that  the  telephone  can  be  readily
switched  from  the  induction  balance  to  the  tele-
phone, or  vice  versa.  When,  therefore,  a  metallic
disc  is  placed  in  one  of  the  coils  of  the  induction
balance,  and  a  noise  is  heard  in  the  telephone,
the  coil  of  the  sonometer  is  shifted  so  that  the
noise  heard  in  this  telephone  is  judged  by  the
ear  to  be  equal,  and  the  comparison  can  then  be
made  by  means  of  simple  calculations.
The  following  table  gives,  in  arbitrary  values,
the  results  of  various  experiments  as  to  the  sensi-
tiveness in  this  respect  of  discs  of  different
metals,  of  various  sizes  and  shapes  :

Silver,  chemically  pure 125

Gold 117

Silver,  commercial 115

Aluminium 112

Copper ico

Zinc 80

Bronze 75

Tin 74

Iron,  ordinary . ,  53

German  silver 50

Iron,  pure 40

Copper,  alloyed •'..-,  40

Lead 58

Antimony 35

Bismuth 10

Zinc,  alloyed 6

Carbon 2

— {Fleming.}

An  inspection  of  this  table  shows  that  the  values
found  for  different  metals  do  not  correspond  with
their  electric  conducting  power,  although,  roughly
speaking,  the  best  conductors  stand  at  the  top  of
the  table,  and  the  worst  at  the  bottom.  The
effects  appear  to  be  dependent  for  their  action  on
the  phenomena  of  magnetic  screening,  for —

(i.)  If  slots  are  cut  in  the  middle  of  the  plate
its  disturbing  action  is  either  removed  or  very
much  decreased.
