420 Scientific Intelligence. 



specific resistance of mercury in B. A. units as there is in the 

 values obtained for the B. A. unit in absolute measure." The 

 author therefore offered the following : — 



(1) That the time is ripe for a new determination of the ohm 

 that shall be final for the practical purposes of the electrical 

 engineer. 



(2) That such a determination can be made by the method of 

 Lorenz, the specific resistance of mercury being obtained directly 

 in absolute measure by the differential method described. 



(3) That the standard coil should consist of a single layer of 

 wire, the coefficient of mutual induction of the coil and disc, 

 circumference being calculated by the new formula. J. t. 



8. Alternating versus continuous currents in relation to the 

 Human body. — At the meeting of the British Association held 

 at Leeds, 1890, a paper was presented on this subject by H. 

 Newman Lawrence and Arthur Harries. They arrive at the 

 following conclusions : 



A. When the human body, with the skin in its normal un- 

 moistened condition, comes into contact for an appreciable time 

 with base-metal conductors of a dynamo-generated continuous 

 current passing at 100 volts in such a way that the current 

 passes from hand to hand, and the total contact area is about 90 

 square centimeters : 



(1) A current of about 0'016 Amperes will pass through it. 



(2) This current can be borne without discomfort for 15 to 30 

 seconds. 



(3) After about 30 seconds unpleasant burning sensations 

 become marked and increase. 



(4) The subject is perfectly able to release himself at will 

 during any portion of the time of contact. 



B. When the human body comes in contact with dynamo- 

 genei-ated alternating currents, alternating at about 60 to *70 per 

 second under the same conditions as above. 



(1) A current of about 0*075 Amperes will pass through it. 



(2) This current is six times greater than that which produces 

 discomfort. 



(3) Instantly the subject is fixed by violent muscular contraction 

 and suffers great pain. 



(4) The subject is utterly unable to release himself, but remains 

 exposed to the full vigor of all the current that may be passing. 



C. When circuit from electric light or power conductors is 

 accidentally completed through the human body, the danger of 

 serious consequences is many times greater when alternating 

 than when continuous currents, are passing at equal voltage, and 

 this is still to a large extent true if the voltage of the continuous 

 current be double that of the alternating. 



D. (1) With both forms of current a reduction of contact area 

 materially reduces the amount of current strength that passes. 



(2) With the alternating current, if the rate of alternation 

 be reduced below 50 per second, the sensations of pain accom- 

 panying muscular fixation will be increased, while if the rate of 



