640 ELECTRICAL UNITS. THE RHEOCORD. 



deduced: I. If there is great resistance in the circuit in the arc of closure, as is 

 the case when a nerve or a muscle is intercalated in a closing arc, the strength 

 of the current may be increased only by increasing the number of electromotive 

 elements. II. If the conduction-resistance in the arc of closure, in comparison 

 with that in the battery, is exceedingly small, an increase in the strength of the 

 current cannot be brought about by increasing the number of elements, but only 

 by an increase in the surface of the plates in the element. 



It is important to differentiate exactly the terms electromotive force and current- 

 strength. The electrical current may be compared with a current of water. The 

 cause of the current in the water is the hydraulic pressure, that of the electrical 

 current the electromotive force. The current-strength is the amount of water, or 

 the amount of electricity, that passes in one second through the transverse section 

 of the conductor. The pump that drives the water to the top of a high vessel, 

 and thus generates the hydraulic pressure, corresponds to the electrical element. 

 In the current of water a mass is set in motion, in the electrical current a force. 



Since 1881 the electrical values, especially current-strength, electromotive 

 force and resistance, have been indicated with reference to units that have a simple 

 relation to the so-called absolute units. Those units are designated absolute that 

 refer to the unit of length (cm.), the unit of time (sec.) and unit of weight (gr.). 

 The unit of resistance is the ohm (= io 9 absolute units). It is equal to the re- 

 sistance of a column of mercury at a temperature of o C., having a transverse 

 section of i square meter and a length of 1.026 meters. The ohm is, therefore, 

 only a little larger than the earlier unit of Siemens (i m. long and i square meter 

 in transverse section) . 



The unit of electromotive force is the volt (= io 8 absolute units). A Daniell's 

 cell has an electromotive force of i.i volts. 



The unit of current-strength is the ampere, that is the current that generates 

 an electromotor force of i volt in a circuit having a resistance of i ohm (therefore 

 o.i absolute unit). An ampere generates 0.174 cu. cm. of exploding gas in one 

 second at a temperature of o and an atmospheric pressure of 760 mm. 



An electrical current in passing through a wire generates heat, the amount 

 of which is proportional to the product of the resistance multiplied by the square 

 of the current-strength, or, according to the law of Ohm, of the current-strength 

 multiplied by^the electromotive force. The product of this from volt and ampere 

 is equal to io 7 work-units and is designated a watt. 



According to the technical designation, i watt equals 0.00136 horse-power 

 (i horse-power equals 75 kilogrammeters) . 



As, in absolute measure, the mechanical heat-equivalent of the gram-calory 

 equals 42,000,000 work-units, ^ or 0.24 gram-calory is generated in a circuit 

 with an electromotive force of i volt and a current-strength of i ampere in a second. 



The density of the current must further be especially distinguished from the 

 current-strength. As the same amount of electricity must always pass through 

 any given transverse section of the circuit, the electricity must obviously be 

 denser at the constricted portions and less dense at the wider portions if the 

 transverse section varies in size. If S indicate the current-strength and q the 

 transverse section of the part in question, the density (d) at this point will be 

 d = S -f- q. 



If the arc of closure of the galvanic circuit be divided at the one pole into 

 two or several circuits, which are reunited at the other pole, the total of the current- 

 strengths is equal to the strength of the undivided current. If, further, the dif- 

 ferent circuits vary with respect to length, transverse section, and material, the 

 current-strengths passing through the wires are inversely proportional to the 

 conduction-resistances. 



According to this principle, that of the derived circuit, the rheocordof du Bois- 

 Reymond is constructed. With the aid of this instrument it is possible to pass 

 from a galvanic current a derived current of any determined strength for the 

 stimulation of a nerve or a muscle. 



From each of the poles (Fig. 224, a b) of a galvanic battery are given off two 

 wires, of which the one pair (ac and bd) pass to the nerve of the frog-preparation 

 The intercalated segment of nerve (c d) offers a high degree of resistance 

 to this branch of the current (a c d b) . The second branch of the current conducted 

 from a and b (a A, b B) passes through a thick brass plate (AB), which is made 

 up of seven pieces lying side by side (1-7) and united through the brass 

 plugs (from S, to S 5 ) placed in the intervals, except between i and 2, so as 

 to form an uninterrupted circuit. It will at once be clear that by means of 



