Early Pointolite Bulb - UK Vintage Radio Repair and Restoration Discussion Forum

Gridiron · 17th Sep 2025, 3:07 pm

I found this at the last Malvern Fleamarket together with a mercury vapour bulb. It has written on it "Royal Ediswan Pointolite Patented 500 CP" and seems to be early 20's in date as it has a top pip seal and the seal wires look to be platinum.
Stanley Mullard first made his name developing this type of bulb when working for Ediswan before he set up on his own, as detailed in the book "The Setmakers ". As the name of the bulb implies, it gives a very intense point light source and needs external ballast resistors to operate, there is quite a lot of information on the net about these bulbs which were made until the 50s.
The Edison screw type base has four connections, the 2 extra concentric rings are for the filament required to start the arcs in the bulb to the bead and plate electrodes, the intense light point does not come from the arc but when one electrode becomes white hot.
Mike.

high_vacuum_house · 17th Sep 2025, 7:06 pm

Very interesting bulb. It works on thermionic emission, The cathode being a pinhead size bead of tungsten which is very heavily bombarded with electrons until it glows white hot. the bead is on a bimetallic arm as it needs to move close to the filament when cold to start working!
As the pinhead size tungsten bead is the light source, this makes it easy to focus and project a near perfect dot rather than a line with a conventional coil filament

Christopher Capener

kalee20 · 17th Sep 2025, 11:52 pm

Super! I hope you can get it working!

(I rather suspect that the mating screw socket with the extra connections might be rarer than the bulb!)

matthewhouse · 18th Sep 2025, 5:38 pm

Fantastic! It's amazing how creative old lighting used to be. I also hope you get it working.

emeritus · 18th Sep 2025, 8:16 pm

There is an illustration of one with a bayonet base in a 1927 school physics book on electricity. No description of the circuitry though. In an old GEC catalogue, bayonet lamp bases were available with three contacts on the base, and the metal collar bearing the pins would have been able to provide a fourth contact. N.B. the letters mentioned in the description do not appear on the illustration, and there are no other drawings of the lamp.

Gridiron · 19th Sep 2025, 9:50 pm

I found two leaflets on line, the earlier one from 1924 shows the bulb I have with others, the other gives information on operating bulbs, which is quite an involved process, the bulb I have requiring a high current 100 volt minimum DC supply and a high current rotary switch and high wattage resistors.
The 7 volt ioniser filament has firstly to be energised and then high voltage applied to the bead electrode to start the arc between it and the filament, then voltage to the plate electrode to transfer the arc to between the plate and the bead, and so I am a bit wary of trying it.
Also would the intensity be enough to be dangerous to view directly?
Mike.

17th Sep 2025, 3:07 pm	#1
Gridiron Heptode Join Date: Apr 2010 Location: West Midlands, UK. Posts: 790	Early Pointolite Bulb I found this at the last Malvern Fleamarket together with a mercury vapour bulb. It has written on it "Royal Ediswan Pointolite Patented 500 CP" and seems to be early 20's in date as it has a top pip seal and the seal wires look to be platinum. Stanley Mullard first made his name developing this type of bulb when working for Ediswan before he set up on his own, as detailed in the book "The Setmakers ". As the name of the bulb implies, it gives a very intense point light source and needs external ballast resistors to operate, there is quite a lot of information on the net about these bulbs which were made until the 50s. The Edison screw type base has four connections, the 2 extra concentric rings are for the filament required to start the arcs in the bulb to the bead and plate electrodes, the intense light point does not come from the arc but when one electrode becomes white hot. Mike. Attached Thumbnails

17th Sep 2025, 7:06 pm	#2
high_vacuum_house Nonode Join Date: May 2003 Location: Belper Derbyshire Posts: 2,243	Re: Early Pointolite Bulb Very interesting bulb. It works on thermionic emission, The cathode being a pinhead size bead of tungsten which is very heavily bombarded with electrons until it glows white hot. the bead is on a bimetallic arm as it needs to move close to the filament when cold to start working! As the pinhead size tungsten bead is the light source, this makes it easy to focus and project a near perfect dot rather than a line with a conventional coil filament Christopher Capener __________________ Interests in the collection and restoration of Tefifon players and 405 line television Last edited by high_vacuum_house; 17th Sep 2025 at 7:12 pm.

17th Sep 2025, 11:52 pm	#3
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,938	Re: Early Pointolite Bulb Super! I hope you can get it working! (I rather suspect that the mating screw socket with the extra connections might be rarer than the bulb!)

18th Sep 2025, 5:38 pm	#4
matthewhouse Octode Join Date: Jul 2006 Location: Willand, Devon, UK. Posts: 1,043	Re: Early Pointolite Bulb Fantastic! It's amazing how creative old lighting used to be. I also hope you get it working.

18th Sep 2025, 8:16 pm	#5
emeritus Dekatron Join Date: Jan 2012 Location: Brentwood, Essex, UK. Posts: 5,691	Re: Early Pointolite Bulb There is an illustration of one with a bayonet base in a 1927 school physics book on electricity. No description of the circuitry though. In an old GEC catalogue, bayonet lamp bases were available with three contacts on the base, and the metal collar bearing the pins would have been able to provide a fourth contact. N.B. the letters mentioned in the description do not appear on the illustration, and there are no other drawings of the lamp. Attached Thumbnails