14 
PROFESSOR B. SARDERSOR OR THE ELECTROMOTIVE 
vertical axis, by working screws, of which the milled heads are conveniently placed for 
the purpose. 
The compensator .—As is well known to every reader of physiological books. 
Professor dtj Bois-Beymoxd has applied the term to an instrument which consists 
essentially of a long thin wire connecting the poles of a standard cell. The wire being 
of uniform thickness, the difference of potential between any two points in its length 
is to the whole difference between the ends, as the distance between these points is to 
the total length. In a compensator so constructed the total difference has to be 
determined by a previous operation. The compensator we employ differs in no 
respect in principle from the potentiometer of Mr. Latimer Clarke* in which the 
total difference is equal to the whole electromotive force of the standard cell, and not, 
as in du Ecus’ compensator, to a certain portion of it. The form we have given 
to the instrument is very different from that of Mr. Clarke’s instrument, and far 
more convenient for our purpose (fig. 5). The compensator wire, or rheochord, has 
Compensator and Reverser. (From a photograph.) 
a total resistance of 5 ohms. On a board, 27 centims. long, is fixed a rail of vulcanite 
1 centim. broad and as much high, and 25 centims. long. Along the upper margin of 
this rail, which is divided into millimeters, runs a wire of platinum, of which the resis¬ 
tance is equal to a \ ohm. Each end terminates in a metal block. A second rail or 
bar of brass runs along the board parallel to the first. It is broken at two points for 
connexion with the end of the coils 1 and 2, of which, the resistances are severally 
and 2| ohms, so that when the plugs are out the total resistance between the ends 
of the bar is 4*75 ohms. The bar is united at one end with one of the terminals of 
the graduated wire. At the other it has a binding screw with two holes. The 
opposite terminal of the graduated wire has two binding screws, one with two, the 
* “ On a Voltaic Standard of Electromotive Force,” Proceedings, 1872, p. 444. 
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