Dimension ( W-D-H ) 110 mm x 115 mm x 50 mm  /  4_1/4" x 4_1/2" x 2"

LABOR  Low-Level DC-AMPERE METER   SIEMENS & HALSKE / Germany (1930)
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CLASS 1   300 μA / 160 mV / 533 Ω    0.3 mA ;  1 mA ;  3 mA ;  10 mA

Housing is made of bakelite  -  Original state:  No seal was broken!

EXTENSION OF THE MEASUREMENT-RANGE WITH A HOME-MADE »TWIN-SHUNT« ...
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                         3 mA  ==>>  30 mA   &   10 mA  ==>>  100 mA

         

                              The right picture shows the inside ...

     Experiments  with  HELIPOTS  &  WHEATSTONE BRIDGE  resulted in:  

     The [  30 mA ]  3 mA Range needed a  21.0 Ω  resistor as shunt,
                                made out of selected  33 Ω // 39 Ω ;
     The [ 100 mA ] 10 mA Range needed a   6.7 Ω  resistor as shunt,
                                made out of selected  12 Ω // 12 Ω .

"How to find the value of a SHUNT ?"
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When you want to expand (eg.) the  10 mA-Range to 100 mA-Range, than
let a 10 mA current go through the instrument:  It shows FULL SCALE.
Now connect  an  EXPERIMENTAL  RESISTOR (= "SHUNT")  parallel to the
instrument until it shows a  10-times reduced  current = 1 mA.  This
is the  value of the searched SHUNT!  ( "Fine tuning" recommended if
another Lab-A-Meter with right ranges is available. )

   

   What "SIZE" (= Watt ) the  SHUNT  has to have ?     ≥  0.1 W  !

   U = R*I ;  P = U*I = R*I2   ===>>  6.7 Ω * ( 9o mA )2  <  70 mW

impressum:
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© C.HAMANN        http://public.BHT-Berlin.de/hamann        11/14/09