148 EMPIRICAL TOOLS AND EMPIRICISM 



Paris. With this standard are compared the kilogram masses main- 

 tained by the various national bureaus of weights and measures; with 

 these masses, in turn, are compared the standards used by commer- 

 cial manufacturers of "weights"; and so on in a kind of apostolic 

 succession. Even as we thus multiply our standards, we multiply our 

 instruments, taking care to assure their mutual consistency by simi- 

 lar cross-comparisons. Generally we venture much further still, bring- 

 ing into play entirely new kinds of instruments that yield us powerful 

 new secondary denotations. 



Calibration. Taking as example the concept "electric charge," I 

 simplify by considering only comparatively large charges passed in 

 D.C. systems. The fundamental unit, the coulomb, is defined (tempo- 

 rarily) as the charge which deposits 0.00112 gram of metallic silver 

 when passed under specified conditions through a specified device: 

 the silver coulometer. Smaller or larger charges are measured as pro- 

 portionately smaller or larger deposits of silver. The specifications of 

 the apparatus and procedure are so clear and complete that the con- 

 cept "electric charge" here acquires all the unequi\'Ocality one hopes 

 to find conveyed by a well-contrived primary denotation. But— as 

 everybody who has used a silver coulometer will testify— operation of 

 the device demands the expenditiire of a good deal of time, effort, 

 and irritation. The silver coulometer is not a highly practical device. 

 Often we prefer to use in its place an instrument as different as, say, 

 some type of integrating galvanometer in which charge is determined 

 by the measurable extent of the mechanical displacements its passage 

 produces. A sophisticated electromagnetic theory may be required 

 first to suggest the possibility of such an instrument. But entangle- 

 ment of this theory with the denotation of "charge" is readily avoided 

 —by the simple expedient of calibration. 



We connect an integrator-galvanometer "in series" with a silver 

 coulometer, so that the "same charge" passes through both. The ob- 

 servable correlation of the results yielded by the coulometer and by 

 the galvanometer supplies the sought-for calibration of that particu- 

 lar galvanometer. From the galvanometric measurement we can then 

 calculate the result that would be given by the less convenient 

 coulometric measurements we need now no longer undertake. Con- 

 tinuing our studies, we arrive ultimately at a general result of still 

 greater value. We find that the values yielded by the coulometer cor- 

 relate with the readings afforded by all galvanometers of the same 



