Conversation so far:
Who was Helen Duncan?
Helen Duncan was a Scottish medium best known as the last person to be imprisoned under the British Witchcraft Act of 1735.
How was Harry Price connected to Helen Duncan?
£50 was paid to the medium Helen Duncan so that she could be examined under scientific conditions. Price was sceptical of Duncan and had her perform test séances.
Did Harry Price investigate other things related to Helen Duncan besides having Helen Duncan perform a number of test séances?
She was suspected of swallowing cheesecloth which was then regurgitated as ectoplasm. Price proved through analysis of a sample of ectoplasm produced by Duncan, that it was made of cheesecloth.

Question: What other involvement did he have with Helen?
Self-contained rewrite: What other involvement did Harry Price have with Helen Duncan besides test séances, and proving through analysis that ectoplasm produced by Helen Duncan was made of cheesecloth?