If the conductor between the battery poles be such that the _external_ resistance _x_ may be practically left out, then C = E / _r_, and no change in the strength of the current will be effected by adding any number of cells in series, as _r_ will increase equally with _E_, and therefore _C_ will remain the same; but if under the same conditions the cells be joined in _multiple arc_, then _r_ will decrease as _E_ increases, and therefore _C_ will be increased. Thus with a short circuit of small external resistance, the strength of the current will be increased by increasing the size of the plates, or by joining the cells in multiple arc, but not in series. If the conductor between the poles of the battery be such that the external resistance _x_ becomes very great, then C = E / (_x_ + _r_), where _x_ is very great compared to _r_. By joining the cells in multiple arc _r_ is decreased, but _E_ and _x_ remain the same, and therefore _C_ is not materially altered, as _x_ is very great compared to _r_. By connecting the cell in series, _r_ is increased, and so is _E_, but as _r_ is still very small compared to _x_, the strength of the current _C_ is increased. Thus with a long circuit of great _external_ resistance, the strength of the current will be increased by joining the cells in series, but not in multiple arc. When the external resistance _x_ is neither very large nor very small in comparison with the battery or internal resistance _r_, then the strength of the current _C_ will be increased by adding the cells in series, and also in multiple arc. By the former process the E.M.F. _E_ is increased more than the resistance of the circuit _R_ or (_x_ + _r_), and by the latter process, the E.M.F. _E_ is unaltered, whilst the circuit resistance (_x_ + _r_) is decreased. All the above may be practically demonstrated by the employment of suitable _galvanometers_. _Frictional Electricity._--_Frictional_ electricity is produced by the friction of two insulators. There is _no difference whatever in kind_ between "Voltaic" and "frictional" electricity. _Comparison with Voltaic Electricity._--The electricity generated by friction possesses a great electro-motive force, producing on even a small conductor a large charge, whereas the electricity generated by the galvanic cell possesses a very small electro-motive force, and produces only a small charge on a small conductor. But when the conductor is large, the electricity produced by the galvanic cell will almost instantaneously charge the conductor to the maximum potential it can produce, the galvanic cell developing an immense quantity of electricity by the chemical reaction; whereas the quantity developed by friction between two insulators is so small, that if it be diffused over a large conductor the potential of the conductor will be very little increased.