I failed a three batches of salt bridges before managing to have one stable made.

The right protocol:

• Mix 200 ml water, 5 g of agar and 75g of salt in a cooking pan.
• Bring it to a boil while mixing regularly.
• Let it boil for a few minutes (not too much, otherwise you will loose your water)
• Pour the boiling mix in appropriate containers
  • Reused glass jars (with an opening having a size close to the widest diameter of the pot)
  • Glass petri dishes
• Let them cool down a bit
• Place them in the fridge for a few hours

What not to do:

• Use hot water from a kettle and mix everything in a cold pot
  • This induces a very uneven mix, where salt and agar are not well dissolved
• Let it sit in the pot beore pouring it on top of the substrate
  • Warm liquid may harm the microbes in the substrate
  • Most of the agar and salt will sediment at the bottom of the pot, and your mix will not become a gelatin
• Do not forget that the fridge step is important !
• I highly suspect gelatin to be inefficient for this process. My successful batch was made from agar-agar from a chemical provider. I think other sources may be fine, but I have not managed to use them properly yet.

Mud samping location - Sint-Jans-Molenbeek, Belgium

Design details:

• The mud sample was taken from the waterland at the bottom of Scheutbos Park in Sint-Jans-Molenbeek.
• Its approximate volume is 50 cl.
• On top is a highly concentrated Petri-dish-sized salt bridge.
• On top is filtered water.
• Electrodes are made of copper from dead batteries. They have an approximate surface area of 100cm2 each.
• Crocodile clips are clipped on the electrodes and brought out of the pot for measurement.

Results:

• Tension is unsensitive to oxygen presence, and is in the 300mV range.
• Current is in the 1-2mA range.
• Power is therefore in the mW range.

Design details:

• The mud sample was taken from the waterland at the bottom of Scheutbos Park in Sint-Jans-Molenbeek.
• Its approximate volume is 3 cl.
• On top is a salt bridge the size of the jar.
• On top is filtered water.
• Electrodes are made of copper from dead batteries. They have an approximate surface area of 10-20cm2 each.
• Crocodile clips are clipped on the electrodes and brought out of the pot for measurement.

Results:

• Tension is sensitive to oxygen presence, and is in the 100mV range when activated, 30 mV at rest.
• Current is sensitive to oxygen presence, and is very low - about 0.3 mV when activated, dropping to 0.1 mV at rest.
• Power is therefore in the microW range.

Design details:

• The substrate is made of vegetable scraps (potatoes, carrotes, zucchinis) covered with compost tea (from FoAM's vermicompost production), and inoculated with a tea spoon of mud from Scheutbos Park.
• Its approximate volume is 7 cl.
• On top is a salt bridge the size of the jar (broken during the manipulations).
• On top is filtered water, which was quickly populated by compost tea from below the salt bridge.
• Electrodes are made of copper from dead batteries. They have an approximate surface area of 10-20cm2 each.
• Crocodile clips are clipped on the electrodes and brought out of the pot for measurement.

Results:

• Tension is sensitive to oxygen presence, and is in the 200mV range when activated, 80 mV at rest.
• Current is sensitive to oxygen presence, and is very low - about 0.4 mV when activated, dropping to 0.1 mV at rest.
• Power is therefore in the microW range.

• Electrode surface area seems an important feature.
• Most of the mud in the Tall-Mud design has been sitting here for a week, which may have an influence on community development.

• Scale-up the vegetable+compost tea design.
• Look for carbon electrodes, which seem more appropriate
• Replace salt bridge with paper