‘Depends on what he’s wearing,’ I said. ‘I think he’s got a drawer full of them alongside the cufflinks.’

‘Sharp dressed man,’ she said. ‘Bit old fashioned for my taste.’

Oh, you don’t know, I thought, but you suspect. And you can just go on suspecting.

‘It’s hard to stay current working in a place like this,’ I said. ‘What’s your mum have on her wrist?’

‘Just skin,’ said Caroline. ‘She says “clock time is an imposition of industrial capitalism and should be resisted if not ignored”. Besides, she thinks it interferes with the flow of energy around her chakra points. And you still haven’t said what all the phones are for.’

‘Somebody,’ I said, meaning Lesley May, ‘has found a way to tap directly into the energy potential of a smartphone.’

‘Does it have to be an Apple or will an Android do?’

‘Anything with a chip-set,’ I said.

‘Wait,’ said Caroline. ‘Are you saying magic doesn’t just destroy the chips – you can actually get power out of it as well? How do you know that?’

Because I’d trained myself to do a very consistent were-light for testing purposes and then measured the output while feeding calculators, phones and, once, an obsolete laptop into the spell. Then I measured it with an antique optical spectrometer that I’d found in a storeroom. It was a beautiful brass and enamel thing that looked like someone had bolted two telescopes to an early turntable. It had taken me another two weeks to find the prism which was in a different box with some notes handwritten in Latin which I hadn’t dared show Nightingale in case he confiscated them. I hadn’t translated them yet, but from the diagrams I was pretty sure the author had been conducting experiments similar to mine.

‘How do you feed a calculator to a spell?’ asked Caroline.

‘Same way you’d do a ritual animal sacrifice, except without the animal,’ I said.

This amazed Caroline, not least because she hadn’t known you could do ritual animal sacrifice – which really shouldn’t have surprised me, what with her mum being her mum.

‘Ten points to Ravenclaw,’ she said.

‘Really?’ I said. ‘I always fancied Gryffindor.’

‘Dream on,’ she said. ‘Definitely Ravenclaw.’

‘I think,’ I said, ‘that it might be possible to trigger a sort of magic chain reaction that feeds off the chipset without the practitioner having to do anything else.’ I nearly said it was like setting a phaser to overload, but I’ve learnt to keep that kind of joke to myself, even with people who make Harry Potter references – especially with people who make Harry Potter references.

‘To do what exactly?’ asked Caroline.