[Solved] Lab trivia: the clamp-on ammeter

Welcome to the second episode of our unico-lab trivia, the first one in English language. The pilot episode was unexpectedly successful, so we decided to go ahead with the series and to enlarge the audience and the fun we followed a colleague advice to write it in English.

The rules are very easy. Look at the picture here below, it is a detail of lab instrument / equipment, try to guess what it is. Write your guess in a comment and for each good one you will get a little help. The winner will be mentioned in the explanation post where the full picture will be revealed along with an explanation of what it is and how it works.

The solution is after the jump...

The current clamp 

Ok, I've to admit that you were too fast and Roberto, again, was answering correctly a minute or two after the publication. He was kind to edit the winning reply with something else to allow also other people to play, but again Paolo and Mattia got it right in no time. This is telling me that I've to make things more difficult and hopefully still exciting if we want to make the game longer lasting and possibly we have to find a way to hide the right reply to others. I'll try to find a technical way out for this, for example, enabling moderation of comments.

But back to the subject. What you see in the picture is the clamp of a current probe, an instrument that is able to measure the amount of electrical current passing through a wire. During the laboratory lessons at the university, you learn that voltages have to be measured in parallel, while currents in series. This means that if you want to measure the voltage drop between two points in a circuit, you can touch these points with the voltmeter leads and read the value on the display. But, if you want to measure the current flowing in a wire, you need the break the wire and let the current pass through your ammeter. Breaking the circuit is not always easy or not even possible, and what you want is something less invasive.

Immagine da Biezl

The solution is to use a transformer as sketched in figure. The red rod is the conductor where the unknown current I is flowing. The ring is the schematic representation of the two halves of the clamp. The alternating current I is generating a magnetic field B into the clamp ring and consequently a given amount of current is generated on the coil windings. This "secondary" current is directly proportional to the unknown one and inversely proportional to the number of turns.

Measuring the secondary current with a standard ammeter (included into the clamp) reveals the value of the unknown current flowing in the conductor.

This is the working principle of the simplest available current probe that is suitable only for alternating currents (AC). To measure DC (direct) current you need a slightly more complicated version that is beyond the scope of this simple post.

A final word. Be aware that playing with the electrical current is deadly dangerous, so in case of trouble call an expert!

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ps. per gli amici e lettori italiani: anche se non avete capito nulla dell'introduzione, sapete già come funziona il gioco. Se volete potete lanciarvi e lasciare il vostro commento in inglese, tanto non può essere peggio del mio, oppure scrivete in italiano che va bene lo stesso :)