254 Electrical Machines. 



the negative conductor, while the ball is approximated to the posi- 

 tive conductor, sparks at least as long, and as frequent, will be ob- 

 tained, as when the negative conductor, or cushion, has the best pos- 

 sible communication with the earth. I conceive that any metallic 

 surface or surfaces, duly connected with either conductor, must be- 

 come virtually a part of that conductor, and partake of its excite- 

 ment. In this predicament, whilst receiving a charge, are the coat- 

 ings of a Leyden jar, or an association of such jars in a battery. 

 The effect of the machine is merely to transfer the fluid from one 

 surface to another. After the conductors, and any jar, or battery, 

 associated with them are charged, there is no more electricity in 

 the surfaces than before ; since whatever one has gained, the other 

 has lost. 



If the impression of the learned professor, were correct, how could 

 a battery or a jar be charged, where both it, and the machine are 

 insulated from the earth ? Yet experience shows that it is under 

 these circumstances that a charge is most easily imparted. When 

 the conductors are in a state of excitement, and both insulated, the 

 one will of course be as much below that of the surrounding neutral 

 medium, and of the great reservoir, as the other is above that stand- 

 ard. When we connect either conductor with the earth, it returns of 

 course to the neutral state of the earth ; but the difference between 

 the excitement of the conductors is sustained by the power of the 

 machine to the same extent as before ; hence the length and frequen- 

 cy of the sparks will not be found to be sensibly altered. It follows 

 that when either of the conductors is made neutral by connexion with 

 the earth, the other will have its excitement as much above or below 

 neutrality, as the sum of the differences between each of the two con- 

 ductors and the terrestrial neutrality when both are insulated. Thus 

 supposing that when insulated, the one conductor is relatively to ter- 

 restrial electricity minus ten, and that the positive conductor is plus 

 ten ; when the negative conductor alone is uninsulated, the positive 

 will be plus twenty, when the latter is alone uninsulated the former 

 will be minus twenty. 



It seems to be a common, though as I believe an erroneous idea, 

 that a spark changes its character with the conductor from which it 

 appears to be taken ; so that when produced by presenting a body to 

 the positive conductor, it is considered as positive, and as negative 

 when produced with the negative conductor in like manner. 



