The psychical researchers Eric Dingwall and Harry Price re-published an anonymous work written by a former medium entitled Revelations of a Spirit Medium (1922) which exposed the tricks of mediumship and the fraudulent methods of producing "spirit hands". Originally all the copies of the book were bought up by spiritualists and deliberately destroyed. In 1923, the magician Carlos María de Heredia revealed how fake spirit hands could be made by using a rubber glove, paraffin and a jar of cold water.
In 1907, Hereward Carrington exposed the tricks of fraudulent mediums such as those used in slate-writing, table-turning, trumpet mediumship, materializations, sealed-letter reading and spirit photography. between 1908 and 1914 the Italian medium Francesco Carancini was investigated by psychical researchers and they discovered that he used phosphorus matches to produce "spirit lights" and with a freed hand would move objects in the séance room.
Senses used by mental mediums are sometimes defined differently from in other paranormal fields. A medium is said to have psychic abilities but not all psychics function as mediums. The term clairvoyance, for instance, may include seeing spirit and visions instilled by the spirit world. The Parapsychological Association defines "clairvoyance" as information derived directly from an external physical source.