Stresses  on  the  Elastic  Properties  of  Steel.  279 
was  tested  in  torsion  to  one-third  the  elastic  limit  by  appli- 
cation of  sand,  troptometer-readings  being  taken.  The 
specimen  while  held  with  this  load  in  torsion  was  tested  in 
tension  to  the  elastic  limit.  A  second  specimen  in  each 
series  (Eb)  and  (Cb)  was  tested  in  torsion  in  the  same  way 
to  two-thirds  the  elastic  limit,  and  while  under  this  load  in 
torsion  was  tested  in  tension  to  the  elastic  limit.  The  third 
specimen  of  each  series,  (Ec)  and  (Cc),  was  tested  in  torsion 
to  the  elastic  limit,  and  while  under  this  load  was  tested  in 
tension  to  the  elastic  limit. 
Friction  of  the  Heads  and  Rollers. — The  friction  of  the 
rollers  attached  to  the  lower  arms  and  bearing  on  the  stationary 
knife-edges  was  measured  and  found  to  be  negligible.  That 
is,  the  amount  of  upward  pressure  due  to  this  friction  (con- 
stant for  these  tests)  was  less  than  could  be  read  on  the  beam 
of  the  100,000-lb.  Olsen  testing-machine. 
The  effective  moment  due  to  the  friction  of  the  ball-bearings 
CI 
and  bicycle- wheels  was  determined  for  the  tension-load  of 
800  lbs.  (corresponding  to  a  load  of  4000  lbs.  per  sq.  in.), 
this  being  the  constant  load  in  tension  used  while  the  torsion 
tests  were  being  made.  The  tests  were  made  by  placing  the 
heads  on  the  platform  of  the  machine,  back  to  back,  and 
compressing  them  with  a  load  of  800  lbs.  and  determining 
the  weight  of  sand  necessary  to  move  the  arms.  The  average 
of  a  number  of  tests  gave  16*42  inch-pounds  as  the  effective 
twisting-moment  due  to  the  friction  of  the  heads  and  bicycle- 
wheels.     This  moment  was  deduced  in  making  calculations. 
It  is  seen  from  the  above  that  the  effect  of  friction  in  the 
tests  already  made  has  been  accounted  for  in  a  definite  way, 
making  the  tests  satisfactory.  The  plan  of  work  includes  the 
determination  of  a  friction  curve  for  the  apparatus.  This  curve 
will  show  the  twisting-moment  lost  due  to  friction  for  any  safe 
load  in  tension, 
Results  of  Tests. — Fig.  3  gives  a  stress-strain  diagram 
of  nickel-steel  in  tension.  The  curve  M  is  a  curve  from 
the  average  of  two  simple  tension  tests  of  the  material,  and 
shows  the  elastic  limit  to  be  56,000  lbs.  per  sq.  in.  The 
three  curves  E  represent  the  behaviour  of  the  material  in 
tension  while  held  in  torsion  :  EA  held  in  torsion  at  one-third 
the  elastic  limit,  Eb  held  in  torsion  at  two-thirds  the  elastic 
limit,  and  Ec  held  in  torsion  at  the  elastic  limit.  The  curves 
show  a  lowering  of  the  elastic  limit  in  tension  ;  a  lowering  of 
about  7  per  cent,  in  the  case  of  Ea,  about  21  per  cent,  in  the 
case  of  Eb,  and  about  63  per  cent,  in  the  case  of  Ec.  When 
the   series  of  tests    contemplated  has  been   completed,  it    i> 
