398  Mr.  Mond  and  Dr.  Wilderman  on  a 
enters  the  terminal  (T),  which  is  in  electrical  contact  with 
the  base  o£  the  instrument,  it  passes  through  the  support  to 
the  friction  wheels,  driving  spindle,  electromagnet,  the  screw, 
and  then,  byway  of  the  insulated  half-nut,  from  here  it  passes 
by  the  return  wire  to  the  terminal  (T/)  which  is  insulated 
from  the  base  of  the  instrument. 
There  is  another  distinctive  feature  in  the  arrangement  of 
our  pen  for  registration,  and  this  is  that  our  pen  can  be  used 
at  the  same  time  both  for  registration  and  calibration.  The 
length  of  the  two  guides  (g  and  g'),  the  greater  flexibility 
of  the  two  flexible  springs  (s  and  *'),  allow  the  pen  to  slide 
on  the  guides  long  distances,  and  for  this  the  electromagnet 
(i)  must  also  be  adjusted  at  its  screw  (n).  By  connecting  the 
terminals  (T  and  T')  with  a  battery,  and  by  using  the 
pendulum  of  a  clock  or  other  suitable  calibrating  arrange- 
ments in  the  circuit  for  making  contacts,  we  are  able  by 
inserting  a  corresponding  resistance  to  adjust  the  current 
passing  through  the  electromagnet  so  as  to  get  for  calibration 
small  deviations  of  the  pen  from  the  normal  spiral  as  desired 
each  second,  or  better,  each  half  minute  or  minute.  By 
having  the  terminals  (T  and  T')  at  the  same  time  connected 
with  another  battery  sending  the  current  in  the  same 
direction,  and  by  having  the  electrical  key  or  the  automatic 
-contact  of  the  rain-gauge,  anemometer,  speed  indicator,  &c, 
we  can  so  adjust  the  voltage  and  the  resistance  as  to  get 
a  greater  current  through  the  electromagnet  and  a  corre- 
spondingly greater  deviation  of  the  writing-pen  from  its 
normal  course,  thus  registering  the  observed  or  recorded 
phenomenon  at  the  same  time.  The  results  obtained  for 
calibration  and  registration  cannot  possibly  be  mistaken  for 
one  another  even  if  both  contacts  should  happen  to  be  effected 
simultaneously,  because  two  intervals  of  calibration  cannot 
possibly  with  our  instrument  be  mistaken  for  one,  and  the 
longer  line  of  registration  cannot  be  covered  by  the  shorter 
one  of  calibration.  Not  only  does  this  dispense  with  a 
second  carriage  and  pen,  but  also  leads  to  more  accurate 
results,  because  we  register  and  calibrate  straight  on  one  and 
the  same  spiral,  not  on  two  separate  ones,  and  we  have  not 
subsequently  to  transfer  the  calibration  from  one  spiral  to  the 
other.  For  the  special  purposes,  where  tw^o  different  kinds 
of  phenomenon  are  to  be  recorded  simultaneously,  e,g.  wind 
and  rain,  or  where  two  observers  have  to  record  simultaneously 
the  same  phenomenon,  such  as  the  transit  of  a  star  in 
astronomical  work,  the  instrument  can  be  provided  with  two 
pens. 
The  great  regularity  and  perfection  of  motion  of  the  spindle 
