The salts of iron quickly cause coagulation of the blood, and the clot plugs the bleeding vessels.
When the blocks are interconnected by the plugs all the coils are short-circuited; but if the plug or plugs are taken out, then a current flowing from one"end of the series to the other is compelled to pass through the corresponding coils.
In series with this set of coils is another set, S, which forms a measuring arm, the resistances of which are generally I, 2, 3, 4, 10, 20, 30, 40, 100, Zoo, 300, 400, woo, 2000, 3000, 4000 ohms. The junction between each pair of coils is connected as above described to a block, the blocks being interconnected by plugs all of which are made interchangeable.
A bridge of this pattern has the advantage that the insertion or removal of a plug in the measuring arm does not tend to tighten or loosen all the rest of the plugs; moreover, there are fewer plugs to manipulate, and each plug is occupied.
To remove this obstruction the water was pumped out while the plugs kept the orifices closed.