Torsional  Stress  in  Solutions  of  Gelatine. 
449' 
vertically  by  means  o£  two  steel  points  at  C  and  E.  The 
brass  projection  F  G  is  perpendicular  to  the  direction  of  the 
cylinder  and  has  a  platinum  point  at  its  end  G  which,  when 
the  cylinder  is  twisted  by  means  of  threads  passing  round  the 
wheel  H,  can  be  brought  into  contact  with  a  fixed  platinum 
plate,  thus  completing  an  electrical  circuit  through  a  Post- 
Office  relay.  The  whole  is  supported  on  a  stand  (figure  '2). 
The  threads  producing  the  couple  pass  over  two  pulleys  K  K 
and  are  attached  to  one  arm  of  a  balance.  From  this  same 
arm  hangs  a  tin  vessel  L_,  containing  water  with  a  layer  of  oil 
on  the  top  to  prevent  evaporation.  The  weight  of  this  vessel 
and  a  certain  amount  of  the  water  it  contains  is  counter- 
balanced by  weights  placed  on  the  other  pan  of  the  balance  SO' 
that  the  tension  in  the  threads  at  any  time  is 
equal  to  the  difference  between  the  two  weights. 
If  necessary,  therefore,  all  the  tension  may  be 
removed  by  drawing  off  sufficient  water  from 
the  vessel  L.  The  water  is  run  off  by  means  of 
a  siphon  through  the  electrically  controlled 
valve  M  into  a  second  vessel  N,  which  is  sus- 
pended from  a  spring,  and  the  weight  of  water 
in  the  vessel  N  at  any  time  is  recorded  on  the 
revolving  drum  0  by  the  pen  P. 
The  current  passing  through  the  platinum 
point  and  plate  before  mentioned  is  far  too 
small  to  itself  actuate  the  water  valve  M,  and 
it  is  necessary  to  introduce  two  relays  Rj  and 
R2  to  effect  a  sufficient  increase  of  current. 
Contact  between  the  platinum  point  and  plate 
causes  the  water  valve  to  open  and  a  small 
quantity  of  water  leaves  L.  This,  however,, 
reduces  the  torsional  stress  in  the  jelly,  a  small 
recovery  takes  place,  contact  is  broken  and  the 
valve  closes.  By  means  of  this  intermittent 
action  the  inner  glass  cylinder  which  is  imbedded 
in  the  gelatine  is  always  kept  in  the  same  posi- 
tion, i.  e.  the  strain  remains  constant;  and  the 
curve  drawn  on  the  revolvino-  drum  oivcs  the 
weight  removed  at  any  time,  and  since  this 
weight  is  proportional  to  the  torsional  stress  its 
value  may  be  taken  as  a  measure  of  the  latter. 
Dimensions  : — The  total  length  of  the  outer 
cylinder  was  about  50  cms.,  and  the  end  of  the 
inner  cylinder   was  about  20  cms.  from   the  bottom  of   the 
former.    In  all  cases  the  length  of  the  inner  cylinder  imbedded 
in  jelly  was  the  same,  viz.  about  26  cms.      The  diameter  of 
D 
A 
W 
