A Physical Medium is someone who has the genetic and molecular capabilities in their body make-up that enables the spirit world to construct something physical from their energy. The spirit scientists withdraw this energy known as ectoplasm, and combine it with the energy that they themselves bring, to manufacture something that can be seen, heard or felt in our physical reality. It must be witnessed by everyone present to be categorized as physical phenomena.

There are various ways this is achieved and the results of it differs greatly and, if the spirit world is using ectoplasm, then it is done in darkness as ectoplasm is light sensitive. Only a percentage of one in a million people will have the right chemical structure to become a physical medium (using ectoplasm) and the likelihood of these people ever becoming aware of this fact is very slim, hence, one reason for its rarity.
However, at the moment there is a great surge in the experimentation of using energy in different ways enabling it to be performed in red light and also in daylight situations so that it can be more widely witnessed and enjoyed, with safer conditions.

Also gaining in popularity are groups of people sitting together to combine their collective energy for the purpose of creating something physical, without the onus of necessarily having a physical medium in their midst, which can only be good for its gaining in popularity.
Physical séances are not parlor games. It's not done for impact or to shock and show-off. It's done through the energy of love. It compliments all other forms of mediumship and is added proof of the continuity of life. It can be the experience of a lifetime!

In the olden days it was much more prolific and popular than today. This is for a few reasons.
1/ Is because it takes so long to develop for this and few people have the time or willpower that it takes to persevere. And it always did, but even more so nowadays with the interference of mobile phones and microwaves etc in the atmosphere.
2/ Is that it can be dangerous for the physical medium if the right conditions are not adhered to.
3/ Is because it is not known about so much because the physical mediums of the present day do not demonstrate to the public because of the danger to their health. In the olden days many mediums were damaged by raids on the séances by police or newspaper photographer trying to prove that it was fraudulent.

A few categories of physical phenomena:

Materialization
Independent voice
Direct voice
Transfiguration
Levitation
Precipitated writing or drawing.

In 1848 in Hydesville, New York State, U.S.A., two young girls, Margaret and Kate Fox, established communication with the spirit of a murdered peddler by a code of rappings. This can also be called percussion. It was proof that not only does the human personality survive death, but that it is possible to communicate. This was the beginning of a surge of physical phenomena across North America, and reached England in 1852.
Since this time there has been a vast array of phenomena produced by the Spirit world. A few of the more well known from the U.S. and the U.K. are:

The Bang Sisters of Chicago were filling auditoriums around 1911, demonstrating precipitated paintings and writings. They could just sit either side of the easel and amazing drawings appeared.
Etta Wreight from the U.S.A: A materialization medium who could have three trumpets and independent voice happening simultaneously.
Margery Crandon from Boston: materialization and independent voice.
Helen Duncan from England: materialization and independent voice.
Leslie Flint from England: independent voice. Probably the most tested medium ever.
Jack Webber from England: materialization and direct voice etc.
Madam Noel: materialization.
Florence Cook: materialization in the late 1800's'early 1900's. she was thoroughly researched, tested and investigated by Sir William Crookes who placed his reputation on the line, for he believed it to be genuine.
Minnie Harrison: full materialization in the 1950's.
Arnold Clare: full materialization around 1930's

Noted modern physical mediums today:
Dave Thompson: materialization, independant voice etc.
Stuart Alexander: materialization, independant voice etc.
Warren Caylor: Full Materialization in white light: independant voice Fully formed spirit materialization (the most comprehensive and clear evidence we have ever witnessed:-0)