58 



LOCALIZED ELECTRIZATION. 



months. Mr. Becker has added to the battery, as a means of ascertaining its 

 state of action, and as a guide to the operator, an ingeniously constructed 

 tangent galvanometer. The battery, when freshly charged, and all connections 



Fig. 17. — Muirhead's battery as arranged at the National Hospital for the Paralysed and Epileptic. 



cleaned, is in its most effective state; but as the strength gradually 

 diminishes, it is essential for the medical practitioner to know the degree 

 of diminution at any given time, in order that he may determine the number 

 of cells to be used. The galvanometer coil has about one unit resisttxnce, and 

 in the foot of the instrument a bobbin is inserted, which has aliout 130 units' 

 resistance, and which has no influence on the needle. This additional 

 bobbin is inserted to make the resistance of the galvanometer somewhat 

 equal to the resistance of the battery itself. The terminals are connected 

 with a simple commutator, so that the current can be made to traverse the 

 galvanometer or not. 



When the battery is in perfect action, it has been found that — 



5 cells give 45° deflection. 



Should five cells give only 22° deflection, the battery would be half its 

 strength (if for the sake of illustration we take the angles for the expression 

 of the strength instead of the tangents), and a higher number of cells should 

 be placed in action, where before five were sufficient. 



To show how additional resistance thrown into the circuit affects the 

 reading of the galvanometer, the following observations were made by 

 Mr. Becker. "When 100 cells are used, then — 



