How did Mandelstam and Papaleksi record the emergence of a current in the coil during its accelerated motion?

How did Mandelstam and Papaleksi record the emergence of a current in the coil during its accelerated motion? How did Tolman and Stewart do it?

In the experiment of Mandelstam and Papalski, the occurrence of a current was recorded using headphones, in which a sound arose due to the arising current. In the experiment of Tolman and Stewart, a galvanometer was used to register the current.

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