CHAPTER XIII. 



ELECTRO-PHYSIOLOGY. 



ELECTRICITY. 

 Electrical Measurements. 



THE system of electrical measurements now in use is founded on 

 the centimeter as the unit of length, the gramme as the unit of mass, 

 and the mean solar second as the unit of time. This is commonly 

 designated as the C. Gr. S. system. 



The ampere is the unit of current; the unit of electromotive 

 force, the volt; the unit of resistance, the ohm. 



The ampere is equal to one-tenth of the C. -Gr. S. unit of cur- 

 rent, or approximately the current of an ordinary Daniell cell through 

 an ohm. The volt is 100,000,000 times the C. G. S. unit of electro- 

 motor force, or approximately the electromotive force of a Daniell 

 cell. The ohm is the resistance of a column of pure mercury 1 

 millimeter square and 1063 millimeters in length, at zero degrees C. 

 To Measure Work. To measure work of contracting muscle, the 

 millimeter-gramme is the unit in the metrical system as that work 

 required to overcome a force equal in weight of one gramme acting 

 through the space of one millimeter. 



Cells or Batteries. 1. Daniell Cell. The first constructed con- 

 tan t battery. It consists of a glass jar filled with concentrated solu- 

 ion of sulphate of copper, bathing an unclosed ring of sheet copper 

 round a porous earthen jar filled with sulphuric acid (1 to 10 of 

 ater), in which is immersed a rod of zinc. The zinc pole is the 

 egative or the cathode, and the copper pole the positive or the anode, 

 nd its electromotive force (E. M. F.) is about 1.07 volts. On ac- 

 unt of the constancy of the battery it is the one chiefly used in 

 aboratories of physiology. 



2. Dry Cells. The just-described wet cell gives off fumes, 

 ntains acids, and must be prepared for use. As the dry cell is 

 always ready and without the preceding disadvantages, it is used 

 extensively in the laboratory. The dry cells are usually modified 

 Leclanche batteries. The Leclanche cell consists of a glass jar con- 

 taining a saturated solution of ammonium chloride, into which an 

 amalgamated zinc rod dips. The zinc is negative and the carbon 

 itive. The plate of carbon is fitted into a porous pot packed with 



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