Posts Tagged ‘electrolytic capacitor


The Best Engineering Exam

It’s tough to examine a job applicant well. Human Resources (i.e, ‘Personnel’) Departments can usually only screen applicants for the relevant buzzwords on a resume. Even managers can have a tough time getting past the buzzwords and unverifiable claims of competence. It is especially unfortunate that a manager with hiring responsibility often does not have any significant expertise with the position being filled. Some folks talk a good game that they have never played well. Such applicants are high-value targets to detect – and avoid.

I worked for a company, Big C, that was a significant manufacturer of force measurement devices. For a while, they were offering a few select Engineers a shot at interviewing Engineering candidates. Maybe they just wanted a way to consume enough time so that the candidate would feel that a long trip had been met with a thorough examination. Whatever the rationale of Big C’s HR Manager, my office partner and I were among the select few.

My office partner and fellow Engineering Conspirator, Big R, is a clever fellow. If we don’t think of something almost simultaneously (see? I’m clever, too!), we will be on the same track rather quickly. Somehow, in a way that I no longer remember, we turned a toy into a test.

Our Toy really was special and unique. It was an electrolytic capacitor which had an interesting defect. Capacitors are the electronic equivalent of buckets: they hold electric charge. An electrolytic capacitor is an especially good bucket. Its electrodes are sheets of thin aluminum foil, chemically and electrically etched to have an immense surface area – about 100 times that of smooth foil. Electric charges are stored along the specially-treated surface, so electrolytic capacitors hold a lot of charge.

A capacitor’s electric charge can be almost completely removed by ‘shorting‘ it – connecting its two terminals together, allowing the charge to flow around and neutralize itself.

Electrolytic capacitors, like all real capacitors, also ‘leak’ a little. Leakage allows charge to dribble away, like water through a hole in a bucket. Electrolytic capacitors hold so much charge compared to their leakage that they can hold much of their charge for over an hour (at room temperature).

Leakage can be monitored by measuring the voltage of a capacitor’s stored charge. An occasional brief voltmeter measurement will track the leakage as the voltage falls to zero.

Our toy (similar to the topmost example in the photo) did not behave like a capacitor. After putting a charge into it, it could be shorted for a few seconds without losing charge. That is, a voltmeter measurement before and after shorting would indicate almost no charge loss. This became our Engineer Test.

We would pull the Toy out of a desk drawer (where it had stayed, undisturbed, since well before the candidate arrived) and give it, and a voltmeter, to the candidate. We instructed the candidate to measure the terminal voltage, short the Toy (although we were careful to refer to it as ‘capacitor’, if anything), then measure the terminal voltage again. The candidate was asked an utterly open-ended question: ‘Whaddaya think?‘.

This question was wonderful in its lack of specificity. Some candidates (including graduates of a well-known area Engineering college) were unaware that the Toy was special. These folks could be gently reminded how capacitors are ‘supposed’ to work, and allowed to try answering a subsequent query: ‘What’s happening?‘.

This question was wonderful for its ability to reveal a mind at work – or to reveal a mind that has sand in its gears. We did not expect a candidate, possibly suffering interview anxiety, to instantly divine the exact answer. We did expect a good Engineer to think under pressure. Their cogitation, expressed verbally, was a revelation of their real competence to do real Engineering.

The vast majority of candidates failed miserably. Few could offer thoughtful observations and suggestions. Eventually, the HR Manager quit sending candidates to us. We accepted so very few, and he felt obligated to at least hire a warm body for an open position.

How about you, my friend? Do you have something thoughtful to offer about Jim & Big R’s Engineer Test? If you absolutely know the answer to its strange behavior, save it for a while. But please! Do offer, in the meantime, some observations or additional meaningful tests.


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Basic Understanding

A nation of sheep will beget a government of wolves.
- Edward R. Murrow

Intellectual Property Notice

All original material Copyright James R. Stone 2010, except where specifically noted. Some images licensed under Creative Commons, or GNU Free Documentation License, or unlicensed and public domain.

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