Question

By what substance is core made up in a solenoid?
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Answer 1

and type of insulator you can use the made up of code of solenoid

Answer 2

Solenoid. A long straight coil of wire can be used to generate a nearly uniform magnetic field similar to that of a bar magnet. Such coils, called solenoids, have an enormous number of practical applications. The field can be greatly strengthened by the addition of an iron core.

Very briefly the answer is :

The core of the solenoid is made of soft iron core.