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In the following circles, th...

In the following circles, the four outer numbers are used to calculate the number in the middle. Can you figure out the rule to determine what number should replace the question mark in the third circle?

Correct answers: 108

The first user who solved this task is Jakubovski Vladimir.

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Electromagnet first exhibited

In 1825, the electromagnet in a practical form was first exhibited by its inventor, William Sturgeon, on the occasion of reading a paper, recorded in the Transactions of the Society of Arts for 1825 (Vol xliii, p.38). The publication showed pictures of his set of improved apparatus for electromagnetic experiments, including two electromagnets, one of horse-shoe shape and one a straight bar. The formed was bent from a rod about 1 foot (30 cm) long and one-half inch (1.3 cm) in diameter, varnished for insulation, then coiled with a single spiral of 18 turns of stout copper wire. In return for the Society's medal and premium, Sturgeon deposited the apparatus in the museum of the Society. Sadly, this was lost after the society's museum was dispersed.«

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