CH. XII.] 



THE ELECTROMETER 



137 



Fig. 164 shows a more convenient form of non-polarisable elec- 

 trodes. 



In order to measure the strength (elec- 

 tromotive force) of such currents, the mere 

 amount of swing of the needle is only a very 

 rough indication, and in accurate work the 

 arrangement shown in fig. 165 must be used. 

 The electromotive force is usually measured 

 in terms of a standard Daniell cell,, The 

 two surfaces of the muscle (M) are led off 

 to a galvanometer (B) ; the needle swings, 

 and then a fraction of a Daniell cell is intro- 

 duced in the reverse direction so as to neu- 

 tralise the muscle current, and bring back 

 the needle to rest. From the Daniell cell K, 

 wires pass to the ends a, b of a long platinum 

 wire of high resistance, called the compen- 

 sator ; c is a slider on this wire ; a and c are 

 connected to- the galvanometer, the com- 

 mutator C enabling the observer to ensure 

 that the current from the Daniell passes in 

 the opposite direction to that produced by 



the muscle. If the slider c is placed at the end (> of the compensator, the whole 

 strength of the Daniell will be sent through the galvanometer and will more than 



FIG. 105. Arrangement for measuring the elec- 

 tromotive force of muscle. (M'Kendrick.) 



I. 



FIG. 106. Lippmann's Capillary Electrometer. (After Waller.) 



1. Pressure apparatus and microscope on stand of which the capillary tube is fixed. 



2. Capillary tube, fixed in outer tube containing 10 per cent, sulphuric acid ; the platinum 



wires are also shown. 

 3. Capillary and column of mercury as seen in the field of the microscope. 



