CAP ILL A A' 1 ' ELECTROMETER 521 



The meniscus should either remain steady or should move only slightly. 

 To ensure that the lack of movement is not due to lack of sensitivity, pass a 

 weak current momentarily from F to E and note whether the electrometer 

 responds with a movement of the mercury from A towards U. A suitable 

 weak current may be generated by attaching a piece of brass wire to the 

 distal end of P and a copper wire to the corresponding part of i'^ and, after 

 opening the s.c. key, touching these wires with different hngers. Reverse the 

 wires and note whether a movement of the mercury in the reverse direction 

 is obtained. 



To Test the Sensitivity of the Electrometer. Place a 2-volt accumulator which 

 is needing to be recharged {i.e. E.M.F. of about 1-85 volts) at Z in the circuit 

 of Fig. 12 with its + pole attached to R and — pole to P. Close the main 

 circuit and open the short-circuiting key for a moment (1 to 2 seconds), 

 observing meanwhile the mercury meniscus. It should move apparently 

 upwards in the microscope (actually downwards). Move the slide-wire Q 

 either towards P or R, and again fully depress the key so completing the 

 electrometer circuit and removing the short between E and F. If the meniscus 

 has a smaller excursion than before, release the key {i.e. short the terminals 

 and open the accumulator circuit) and move the slider a little more in the 

 same direction as before. (If the excursion of the mercury, on the other hand, 

 were increased, the slider would have to be moved in the opposite sense.) 

 Continue the process of moving Q, and completing the electrometer circuit 

 till a point of balance between P and R is reached. A sensitive electrometer 

 should show a deflection of 2 to 4 scale divisions in the eye-piece micrometer 

 when the slider is 1 mm. from the point of balance. From this division of the 

 potentiometer wire can be calculated the potential at the mercury surface in 

 the bulb, or if an accumulator of unknown strength is placed at Z and a 

 standard cell {e.g. cadmium cell) is placed between P and E to give a negative 

 potential to the mercury in T, one can measure the E.M.F. of the unknown as 

 follows. Since the combined resistances of the leads are negligible compared 

 with the resistance of the potentiometer wire P R, there exists the same 

 difference of potential between the points P and R as between the terminals 

 of the unknown source of E.M.F., i.e. E^,,. = E.M.F. of the accumulator. 



When the point of balance has been obtained with Q, the potential difference 

 between Q and P {= Ep^^) is equal to the E.M.F. of the cell E{cnd). 



Now En^ : £'pR = Length PQ : Length PR, 



PP 

 E.M.F. of accumulator = E{cad) -^ 



For example, if ^(cad) ^ 1-0185 volts and PR = 1,000 mm. and PQ = 



550-5, then E{acc) = i^^^t^ = 1-85 volts. 



550-5 



If the electrometer is used to find the point of balanced E.M.F. on a Wheat- 

 stone bridge (Experiment 22 (d) ), it is essential to arrange that the direction 

 of the positive current is such that the capillary mercury is the cathode, 

 otherwise mercurous sulphate might he formed in the capillary tube on 

 account of the passage of a rather large current. For the same reason, the 

 short-circuiting key should only he opened for as brief periods as possible. 



Fine instruments may be purchased filled, adjusted and sealed ready 

 for use (Figs. 12 and 106). 



