Which bulb fits this lamp?

2The code: a letter and a number

Signify, who make the Philips bulbs, put it in one sentence: the first letter is the type of cap, and the number is the diameter of the base in millimetres, or the distance between the pins. E is Edison screw, after the man. B is bayonet, the push-and-twist with two pegs on the side, which Britain kept when the rest of Europe went over to the screw. G is a pin cap, and the letters after the G (GU, GY, GX) are variations on the pins that you don’t need to know, because the number does the work: GU10 pins are 10 millimetres apart, GU5.3 pins 5.3, G9 pins 9, G4 pins 4. R is the recessed contact on the end of a linear halogen tube, S a striplight. A small letter at the end, d or q, is how many contacts the base has, two or four, and it changes nothing about what fits in a house.

3The common eight

B22, the bayonet, is the British ceiling: most pendants and older table lamps take it. E27, the big screw, is the rest of Europe, everything IKEA has ever sold, and more British fittings every year as the shops stock what the factories make. E14, the small screw, and B15, the small bayonet, are the candle bulbs in bedside lamps, chandeliers, wall lights and cooker hoods, and they look alike until you turn them over. GU10 is the mains downlight: two fat pins with feet, push in and twist. GU5.3 is its 12 volt cousin, two thin pins that push straight in, and the bulb on the end of them is the MR16, named for the width of its glass face in eighths of an inch, with a transformer somewhere in the ceiling feeding it. G9 is the small mains capsule with two looped wires. G4 is the tiny 12 volt capsule you find in a cooker hood and swear at.

4Measuring an empty holder

With the old bulb in your hand the code is on the metal, small, near the glass. Without it, measure the holder. Screw and bayonet: across the inside of the holder at its widest, which is the width of the cap it takes, and 14, 22 and 27 millimetres are far enough apart that a ruler can’t get it wrong. Pins: from the centre of one hole to the centre of the other, and note whether the holes are round, for a push-fit, or keyhole-shaped, for a twist. Tubes: the distance between the spring contacts, 78 or 118 millimetres for a linear halogen and 221 or 284 for a striplight; for a fluorescent tube, the width of the tube itself, 26 millimetres for the old T8 and 16 for the slim T5.

The cap is the first decision, not the last. The shape of the glass (candle, golf ball, the pear-shaped GLS, a spot) is the second, and has to fit the shade. The lumens, which is the brightness, is the third, and there’s a sheet for that.

5The 12 volt trap

Two spotlights that look the same from the floor: one is a GU10 on the mains, the other a GU5.3 on 12 volts from a transformer in the ceiling void. Signify’s tell is the pins, fat with feet for the GU10, thin and straight for the GU5.3, and the face of the GU10 is a shade smaller. An LED MR16 in an old halogen transformer is the one swap that regularly flickers, buzzes or won’t light, because the transformer was built for a row of 50 watt halogens and an LED is a tenth of one. The fix the trade uses is to swap the fitting for a GU10 and take the transformer out, which is why GU10 is what every new kitchen gets. Same story with G4 and GY6.35 capsules: 12 volts, a transformer, and an LED that may or may not get on with it. Read the label on the fitting before you buy, and if it says 12 V, buy the LED that says it works with your kind of transformer, or buy the GU10 fitting.