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Otto Wolff, from 1900 to 1909, produced the Reichsanstalt-type of resistance standards used at the National Bureau of Standards (NBS.) At the turn of the last century, many precision measurements had to be calculated because the measurement couldn't be made directly since many of the instruments we take for granted today hadn't been developed.A known precision resistance allowed one part of the equation to be selected and the remainder was then calculated.Today there is a Leeds & Northrup doing business out of the UK specializing in industrial furnace temperature monitoring devices. One of Edward Weston's products was "The Weston Standard Cell" which was a mercury-cadmium battery that provided a very specific DC voltage that didn't change over long periods of time. Cell voltage was nominally 1.018638vdc for most of its life.
Central Scientific Company was located in Chicago, IL and generally built various types of devices for laboratory experimental use, mainly for schools, colleges and universities.Rogers Nov, 2014 These two precision resistance boxes were built in Berlin by Otto Wolff and date from around 1900. Biddle of Philadelphia imported these instruments and each box has his tag attached.Biddle was also a very early major builder and supplier of wireless equipment with Reginald Fessenden being one of his prominent customers over a long period of time.Most of these types of devices used nickel-chromium wire wound on ceramic forms.All of these rheostats will have a slide adjustment to vary the value of resistance (or voltage when connected as a potentiometer) as necessary for the experiment.
Also, these types of rheostats were available in many different values of total resistance and maximum current flow ratings.