Taking the Measure

How low must an instrument go to be classed as an electrometer, and has the definition evolved over the years? These questions occurred to me as I perused a blog post from EDN technical editor Paul Rako.

But first, let me back up. Paul has an excellent article on measuring nanoamperes. If you haven’t read it, please do. He provides helpful hints from Jim Williams, a staff scientist at Linear Technology, and Paul Grohe, an application engineer at National Semiconductor.

But in the course of the article, he quotes Grohe referring to the “Keithley 2400 electrometer.” That prompted a response from John Tucker, lead marketing engineer for nanotechnology at Keithley, who noted that the 2400 is a source-measurement unit and not an electrometer, a class of instruments to which Keithley’s Model 6157A belongs. Similarly, Andy Brush, CEO of Tegam, commented that although the Keithley Model 2400 can resolve better than nanoamperes, it does not deserve the title of electrometer. Paul covers this in the blog post I mentioned earlier.

So what, I wondered, is the exact cutoff point at which an instrument becomes an electrometer, and has this changed over time? I pulled out Wedlock and Roberge’s 1963 Electronic Components and Measurements and found Keithley’s Model 410A picoammeter prominently displayed, but no electrometer appellation was attached to it.

According to  Wedlock and Roberge, “Electrometers are voltmeters which feature input impedances as high as 10¹⁶ ohms, and are therefore useful in making voltage measurements from very high-impedance sources.” (They provide as an example Keithley’s Model 610C.) No mention of current at all—it’s a charge and voltage measuring device, as is the foil-leaf electroscope from which the electrometer evolved.

A search of our www.tmworld.com Website brings up lots of references to electrometers (for example, Dale Cigoy, a Keithley applications engineer, refers to “electrometer ammeter” here, bringing current into the electrometer picture), but I can’t find a specific definition. I can’t find one on Keithley’s site either, but I may not have looked enough.

So I tried Wikipedia, which says, “An electrometer is an electrical instrument for measuring electric charge or electrical potential difference.” That’s pretty much in keeping with the Wedlock and Roberge definition. But it adds, “Solid-state electrometers are often multipurpose devices that can measure voltage, charge, resistance, and current. They measure voltage by means of 'voltage balancing,' in which the input voltage is compared with an internal reference voltage source using an electronic circuit with a very high input impedance (of the order of 10¹⁴ ohms). A similar circuit modified to act as a current-to-voltage converter enables the instrument to measure tiny currents of the order of one femtoampere.”

That definition works for me, but I invite others. Use the comment link to respond.