CH. XI.] THE ELECTROMETER 121 



neutralise the muscle current ; if c is half way between a and />, half the Darnell's 

 strength will be sent in ; but this is also too much ; ac will be found to be only 

 quite a small fraction of ab and this fraction will correspond to a proportional 

 fraction of the electromotive force of the Daniell cell. 



Lippmann's Capillary Electrometer. This instrument is often used instead 

 of the galvanometer. It consists of a glass tube drawn out at one end to a fine 

 capillary and filled with mercury. It is connected to an apparatus by which the 

 pressure on this mercury can be lowered or increased. The open capillary tube is 

 enclosed within another tube filled with 10 per cent, sulphuric acid. Two platinum 

 wires fused through the glass, pass respectively into the mercury and the acid, and 

 the other ends of these wires are connected by electrodes to two portions of the 

 surface of a muscle. The capillary tube is observed by a microscope (see fig. 139). 

 The surface of the mercury is in a state of tension which is easily increased or 

 diminished by variations of electrical potential, and the mercury moves in the 

 direction of the negative pole. 



If the shadow of the mercurial column is thrown upon a travelling sensitive 

 photographic plate, photographs are obtained which show the electrical variations 

 in a living tissue in a graphic manner. The instrument is exceedingly sensitive, 

 and its indications are practically instantaneous. The instrument has been much 

 used in investigating the electrical changes of the heart, as will be more fully 

 discussed when we are considering that organ. 



The String Galvanometer. This is even more sensitive than the capillary 

 electrometer, and we owe its introduction to Professor Einthoven of Leiden. In 

 the ordinary galvanometer, as we have seen, the current passes through a fixed 

 coil of wire, and deflects a small magnet suspended in the centre. This arrange- 

 ment can be inverted, the magnet being made large and fixed and the coil small 

 and movable. The Einthoven galvanometer is a development of this type. The 

 magnet is large, and shaped so as to give a very intense field, and the "coil" is 

 reduced to a single thread of quartz, silvered on the surface so as to conduct the 

 current. The movements of this thread are too small to be followed by the 

 unaided eye, so the poles of the magnet are pierced by a hole in which a microscope 

 magnifying 600 diameters is arranged, so as to cast the image of the string on a 

 moving photographic plate. We have already seen one use of this instrument in 

 the study of voluntary muscular contraction (see p. 106) ; we shall have to return 

 to it in our study of the heart, and respiration, in later chapters. 



We can now pass on to a consideration of results. 



In muscles that are removed from the body, it is found that on 

 leading off two parts of their surface to a galvanometer, the galvan- 

 ometer needle generally swings. The most marked result is obtained 

 with a piece of muscle in which the fibres run parallel to one another, 

 and the longitudinal surface is connected with one of the cut ends 

 by a wire (2 in fig. 140). 



On the course o f jhe wire a galvanometer indicates that a current 

 flows from the centre to the cut end outside the muscle, and from 

 the cut end to the centre inside the muscle. If, now, the muscle is 

 thrown into tetanic contraction, the needle returns more or less 

 completely to the position of rest. 



Du Bois Keymond, who first described these facts, called the first 

 current the current of rest, and the second current the current of 

 action; the change in direction is indicated by the expression 

 negative variation ; this means that the current of action is in the 

 opposite direction to the current of rest, and therefore lessens or 

 neutralises it. The word negative is therefore used in its arithmetical, 



