602 



SCIENCE. 



[N. S. Vol. XXII. No. 367. 



another (C. afjinis) which may descend to a 

 depth of at least 1,300 fathoms. 



Theo. Gill. 



electrometer for the stage op the 

 microscope. 

 The capillary electrometer consists of a 

 vertical tube drawn out at the lower end into 

 a fine capillary and filled with mercury (Figs. 

 1 and 2). The upper end of the tube is 

 joined to a cylinder in which a piston is 



Fig. 1. 



Fig. 2. 



moved by a screw, thus making pressure on 

 the mercury column. The end of the capil- 

 lary dips in a reservoir containing 20 per cent, 

 sulphuric acid. A little mercury is placed in 

 the reservoir. Platinum wires lead from this 

 and the mercury in the capillary to convenient 

 binding posts. When mercury is placed in 

 the vertical tube it enters the capillary until 

 the weight of the column of mercury is bal- 

 anced by the surface tension. If the capillary 

 be now dipped in the reservoir containing the 

 sulphuric acid and the piston driven upward 

 by its screw, mercury will be forced out of the 

 capillary into the acid, and on lowering the 

 pressure the mercury will retreat within the 

 capillary, drawing the acid after it. As the 



mercury in the capillary is kept from falling 

 by the surface tension, it is obvious that what- 

 ever increases or diminishes the surface ten- 

 sion, for example an electric current, will 

 raise or lower in corresponding measure the 

 mercury in the capillary. The alteration in 

 surface tension is accompanied by the move- 

 ment of ions between the meniscus and the 

 remaining electrode of the electrometer (the 

 mercury in the acid* reservoir). In practise 

 it is found that this movement can be neither 

 very rapid nor long -continued, without in- 

 juring the sensitiveness of the instrument. 

 The potential difference from even a single 

 element (Daniell or dry cell) is far too large 

 to be used safely. It is advisable to employ 

 a potential divider, or rheochord, which shall 

 permit only a fraction of the original poten- 

 tial (not more than 0.1 volt) to reach the 

 electrometer. 



The electrometer should be kept short-cir- 

 cuited, except during an observation, so that 

 the capillary and the mercury in the reservoir 

 may always be connected through a conductor. 

 The short-circuit key is shown in Fig. 1. 

 A strip of spring brass connected with one of 

 the binding posts of the electrometer rests 

 against a second piece of brass connected with 

 the other binding post, except when depressed 

 by the finger. The point of higher potential, 

 when known, should always be connected with 

 the capillary. 



When the capillary electrometer is con- 

 nected with two points of unlike potential the 

 meniscus is displaced. The pressure neces- 

 sary to bring it back to its original position is 

 proportional to the electromotive force that 

 displaced the meniscus. _ Thus by connecting 

 the electrometer with known differences of 

 potential it may be experimentally graduated. 

 In practise, the relation between the pressure 

 and the potential must frequently be rede- 

 termined. It is usually easier to measure 

 differences of potential, such as the demarca- 

 tion current of nerve or muscle, by compensa- 

 tion. In this method the electromotive force 

 of the demarcation current is measured in 

 fractions of a Daniell cell, or any other con- 

 stant element, by bringing into the same cir- 



