82 EMPLOYMENT OF ELECTRKJITY 



it is, that scarcely any two instruments of different 

 manufacturers correspond precisely in their phy- 

 siological effects. Some machines have no means 

 of modifying their electrical effects : it is needless 

 to say that these are most objectionable, since 

 they afford no facilities either for adapting the 

 power to the constitution or temperament of 

 the patient, or other circumstances of the case, — 

 other machines have the power of modifying the 

 action of the inducing magnetism by the partial 

 withdrawal of the iron core. This plan is open 

 to the objection that it affords no means of pro- 

 ducing a series of delicate and equal increments 

 of power, and it involves also the use of a second 

 magnet for actuating the vibrating spring, and 

 this requires generally the employment of a large 

 battery, thereby increasing the bulk and complex- 

 ity of the machine. 



A third mode of moderating the shock, is by 

 employing the whole coil on all occasions, but 

 modifying the amount of effect, by causing the 

 shocks to pass through variable lengths of an 

 imperfect conductor, such as water contained in 

 a tube. The contrivance is ingenious, but still 

 it does not admit of that definite and accurate 

 degree of graduation which is desirable when 

 accurate comparisons of experiments are to 

 be made ; for the conducting power of water not 

 only varies with its chemical character, but with 

 its temperature. 



