Back in the seventeenth and eighteenth centuries, coffee houses were places to go for debate, discussion or even to learn something new. The Grecian was known for science. Maths instruction (particularly for gambling) could be found with Abraham de Moivre (1667-1754) at Old Slaughter’s on St Martin’s Lane. Other coffee houses were meeting centres for literature, politics, philosophy or even espionage*. Coffee houses became known as “Penny Universities”. The Black Penny on Great Queen St is a café that wants to continue this tradition, with a downstairs “seminar pit” ready to host such discussions. Although the events page still says “coming soon”, if the events do indeed come, this is very much something that’s worth keeping an eye on.
Even without the seminars though, The Black Penny is definitely worth a visit. Entering from the street, the bar is on the left and is stocked with a good looking selection of cakes. We were shown through to the relatively large, bright and airy seating area at the back where a jar of water (infused with cucumber and mint) had been put on the table for us. I had a very good long black and a lovely apple and blackberry muffin with which to take in my surroundings. The muffin was confidently asserted to be nut-free, and so the Black Penny gets a tick in the ‘good nut knowledge’ section on the Daily Grind. The coffee beans were roasted by the Black Penny themselves and while it still says that they serve ‘Alchemy’ coffee on their website, this no longer appears to be the case.
Inside, there are some very interesting architectural features to notice, the remains of a ceiling for example (now removed to reveal the roof) and the acoustics introduced by the speaker positioning. Downstairs in the seminar pit there is apparently a very old stove, though I didn’t get to see that on my visit. However, what immediately struck my eye was what appeared to be a series of coat hooks that looked very similar to a well known brand of battery. Quite what these hooks were for or why they looked like batteries I didn’t manage to ascertain, however, it did get me thinking, can you use coffee-power to light an LED?
You may have heard of a potato battery, or a lemon battery. These are often used in science outreach experiments in schools to demonstrate electricity, or the concepts of current/voltage. Made from an ordinary potato (or a lemon), a copper wire is stuck into one end of the potato and a different metal (usually zinc) is stuck into the other end of the potato. At the Black Penny, there were three things left on the table. My coffee, the mint and cucumber infused water and the tea of my accomplice in many of these reviews (I’d eaten the muffin). Which of these would perform better as a battery?
Although people suggest using galvanised screws as the source of the zinc electrodes, I didn’t have many of those to hand and so had to manage with aluminium foil for one electrode, copper wire for the other. By putting the aluminium on one side of a shot glass, the copper wire on the other and then filling the glass with coffee, I was able to get 0.5-0.8V across the electrodes when I measured it with my digital multimeter (DMM). Fantastic you may think, almost an AA battery, but then if you were to measure the voltage across the water rather than coffee, you will find that you get a voltage of 0.6-0.7V. The result for tea was, perhaps unsurprisingly, about 0.6V.
But voltage is not the whole story. A battery does not just supply a voltage, it gives a current. The current depends on the electrical conductance of the liquid that the electrodes are in. In the case of the potato or the lemon battery, the acid (phosphoric or citric respectively) means that there are free hydrogen ions in the ‘battery’ between the electrodes which mean the electric current can flow through the circuit. Coffee consists of many acids (chlorogenic, quinic, citric etc etc.) and so it seems sensible to ask if coffee could be used to produce a battery with a current that could power an LED? LEDs require both voltage and current, (1.6V and 10mA for the LEDs used here). Hooking up a series of coffee battery-cells meant that, by 6 ‘cells’, I had 3V across the contacts. However the electric current through the coffee battery was very low (the maximum current I recorded using the low acidity Roasting House Sierra de Agalta Honduran coffee prepared in a cafetière was 155 μA). Although this was higher than the current through water (max 81 μA), it is much lower than the current through white vinegar (770 μA under the same conditions). Consequently, in order to light the LED connected to my coffee battery, I had to add salt to each coffee cell which serves as a way of massively boosting the current through the coffee (salt forms a solution of Na+ and Cl- ions that conduct electricity through the coffee). Though even then, my LED only lit dimly and intermittently.
Sadly then, I do not see coffee power as a future for lighting in our cafés, (unless you want to use bulletproof coffee with salted butter). However, it has started to make me wonder, could we use a single coffee-cell to monitor the acidity of our coffee? If you find a method of brewing or a particular coffee especially acidic, it should produce a higher current for the same voltage through the cell, or equivalently, the resistance of the coffee-cell should decrease as the acidity of your coffee increases. Although obviously, it would be a bad idea to drink the coffee after putting it into a cell with copper and zinc (or aluminium) electrodes, you could pour a small amount of your coffee into a shot glass to test it while you were drinking the rest of the coffee. I intend on testing this hypothesis over the next couple of weeks but in the meanwhile, if you have thoughts on this to share (or the results of your experiments), please let me know either via the comments section, email, Facebook or Twitter.
The Black Penny is at 34 Great Queen St, WC2B 5AA
* A history of coffee houses can be found in “London Coffee Houses”, Bryant Lillywhite, (1963)