In 1752,Benjamin Franklin tied a key to a kite and flew it in a storm with the aim of finding out that lightning was a form of electricity.

In 1752,Benjamin Franklin tied a key to a kite and flew it in a storm with the aim of finding out that lightning is a form of electricity.

两种时态都对。一般过去时，重点不在考虑是否“客观真理”，只是为了“时态呼应”；一般现在时，着重暗示这是一种客观现象，不受时间制约，给人感觉是为了寻求真理。