Dr. A. Schuster on Unilateral Conductivity. 255 



easy in one direction through the air than in the other. It is 

 also known that metals condense air in great quantity at their 

 surface ; and if we screw two wires with their condensed air to- 

 gether, it is quite conceivable that particles of air will separate 

 the two surfaces of copper, and that a small voltaic arc will there- 

 fore be formed. Unilateral conductivity would be the result. 

 If we screw a wire which has air condensed on its surface to a 

 binding-screw, part of the air will pass from the wire to the 

 binding-screw; and it would thus be explained that the tempo- 

 rary addition of a new wire may produce a unilateral conductivity 

 in a circuit which has not shown it before. 



V. Experiments confirming the Theory. 



Many minor coincidences seemed to confirm this theory. 

 Cleaning the ends of the wire with the knife generally destroyed 

 the effect. It was, as a rule, observed in those parts of the cir- 

 cuit which had been disconnected over night. It is always easy 

 to find out in what part of the circuit the effect has its seat. 

 We have only to change the connexions in various places, and 

 to observe in what direction the needle is deflected. I mention 

 one particular case. 



The rotation of the magnet one day caused a permanent de- 

 flection of the needle of 295 divisions of the scale. On reversing 

 the wires at the ends of the induction-coil, the needle was de- 

 flected towards the other side. The effect, therefore, had its 

 seat in the induction-coil. The coil was divided into two halves, 

 which were connected by means of a stout copper wire about 

 half an inch in length. I remembered that this piece of wire 

 had been exposed to the air over night, and I therefore reversed 

 the wire \ the needle was deflected 295 divisions of the scale to 

 the other side, showing that my supposition had been correct, 

 and that this small piece of wire, the resistance of which may 

 have been about the hundred-thousandth part of the whole 

 resistance, had caused the deflection. On reversing the wire 

 again, the effect had disappeared. 



Another wire was now taken to join the two halves of the in- 

 duction-coil ; a permanent deflection of about 80 divisions of 

 the scale was observed. On cleaning the ends of the wire with 

 a knife the effect disappeared. 



These experiments seemed alone sufficient to prove the theory. 

 In order, however, to subject it to a severer test, I thought of 

 condensing air artificially on the surface of the wire. This can 

 readily be done by means of powdered charcoal, which, as is 

 known, absorbs air in great quantity. A wire which was in its 

 normal state was therefore laid with one end into powdered 

 charcoal for about five minutes. When reintroduced into the 



