ACTION OF THE EXCISED MAMMALIAN HEART. 
235 
§ VII. Electrometer Indications. 
An examination by the electrometer of the electromotive changes, as revealed by 
the galvanometer, is of obvious importance. The ca,pillary electrometer of Lippmann 
gives indications which follow rapid changes of potential far more faithfully than do 
the indications of the galvanometer, whether “dead-beat ” or freely oscillating. We 
arranged our connections according to the following diagram :— 
The heart is led off at A and B. By the two short circuiting keys, K : and K 2 , its 
current can be sent either through the electrometer E or through the galvanometer G. 
The current is compensated from the battery and rheochord R (its key is not repre¬ 
sented in the diagram). The electrometer E is fixed on the stage of a microscope, 
the tube of which projects into a dark chamber, and the image of the field of the 
microscope, with the capillary column of mercury, is thrown upon a sheet of ground 
glass, on which its movements can be observed through an aperture in the dark 
chamber. The movements of the electrometer are recorded, when desired, by 
substituting a travelling sensitive surface for the ground glass, all light, with the 
exception of that passing through the capillary, being shut off from the plate by a 
screen in which a vertical slit is cut. Our photographs, except where otherwise 
stated, wmre taken with a -^-inch objective, and the image w 7 as formed 90 cm. behind 
it. We made use of sunlight for our photographs, reflected from the ordinary 
sub-stage mirror of the microscope. 
The movements of the mercury in the capillary—advance and retreat—are in the 
same direction as the direction of current; hence, when A is negative to B, the 
movement of the mercury is northward's in the field of the microscope, and, the column 
being arranged to block the light focussed through the capillary on to the travelling 
sensitive surface, this movement appears on the photographic negative as a white 
projection into a darkened area; the reverse occurs if B becomes negative to A, and 
2 h 2 
