For  two  of  the  tubes  used  in  these  experiments  com-
parisons were  made  of  the  resistance  at  0°  with  that  in
water  at  the  temperature  of  the  room,  which  was  about
13°  C.  It  was  found  that  the  mean  difference  of  resist-
ance for  1°  per  unit  of  the  resistance  at  0°  was  '000861.
This  is  of  course  the  change  of  resistance  of  the  contents
of  a  certain  glass  tube,  and  not  to  be  confounded  with
the  temperature  variation  of  the  specific  resistance  of
mercury.
Standard  ohms  have  been  made  in  mercury,  by  using  Forms  of
tubes  bent  so  that  the  requisite  length  is  obtained  in  a  standard
compact  form,  but  they  are  not  very  convenient  in  use,  Ohm.
and  are  of  course  liable  to  breakage.  A  copy  of  the
standard  ohm  can  however  be  easily  made  when  the
resistance  of  a  column  of  mercury  of  definite  cross-
section  and  length  has  been  accurately  found.  Figs.  84
and  85  show  such  copies.  Fig.  84  is  the  usual  form  of
the  standard.  It  is  made  of  platinum-silver  wire,
wound  within  the  lower  cylinder.  The  space  within  up
to  the  top  of  the  wider  cylinder  is  filled  with  paraffin-
wax.  The  ends  of  the  coil  are  attached  to  two  thick
electrodes  of  copper  rod,  bent  as  shown  and  kept  in
position  by  a  vulcanite  clamp.  The  ends  of  these  when
the  coil  is  used  are  placed  in  mercury  cups  in  the
manner  already  explained,  and  should  always,  before  the


394


COMPARISON  OF  RESISTANCES.
