OKLAHOMA ACADEMY OF SCIENCE 135 



gp,M ^.4- t-^-c o'lfc^t t^yj. i|- jc; pQ^ th 6 purposc of this paper to do 

 away with the primary battery since practically a!l of the principle^ 

 Ol c.ectricity ran be demonstrated by its use. The history and 

 development of electricity is very closely related to its study, and 

 yet, ufefnl as it is, it has its limitations. Both teachers and pupils 

 are constantV wiihinq- for a pood strong current to work with and 

 it is to these that this paper is addressed. 



Since practically everv school is provided with alternating cur- 

 rent it is natural that it should be looked to as a source of power. 

 To make this power avai'ab'e, however, two important changes 

 are involved : that of rendering the current direct, and that of 

 reducing the voltage to a value suitable for use in the laboratory. 



Various devices have been m.ade for this purpose, the most com- 

 mon being the motor-generator, the electrolytic rectifier, and the 

 vacuum tube rectifiers ; the best examples of the last are the 

 mercury arc rectifier and the tungar rectifier. The so-called recti- 

 fiers are all alike in that they take the alternating current and trans- 

 form it into a unidirectional pulsating current. A single rectifying 

 unit al'ows one pulsation to pass through and stops or strangles the 

 other, that is to ;•a)^ the current can pass in one direction but not 

 in the other. When both halves of each cycle are desired it is 

 necessary to com.bine two or m.ore units. 



Tn the case of the motor-generator we have an entirely differ- 

 ent type of change. The A. C. current is sent into an A. C. motor 

 which either by belt or rhaft, turns the armature of a D. C. generator 

 which delivers a current of practically constant value. For all 

 uses, for durability, and for cost of up-keep, the motor-generator 

 set is the most satisfactory — the first co:t is practically the on!y 

 cost. If handled properly it will take care of the needs of a 

 laboratory for years and years. The cost of a set installed usually 

 runs from $?00.00 up to $1000.00 or more depending upon the capa- 

 city desired. For a majority of the smaller high schools the price 

 is prhibitive ; often times they have not much more than that to 

 expend upon their entire equipment. For this type of school the 

 electrolytic rectifier or Nodon Valve commends itself. Its cost is 

 sm^all and with little care it can be used almost' indefinitely. 

 The writer made one for $3.00 which he used for ten years in 

 connection with classes totaling from 100 to ISO students each year. 

 The supply houses put out one that can be purchased for $25 00; 

 it is a 'ittle more convenient than the home-made type is likely 

 to be. 



The Nodon Valve consists of a lead-aluminum cell with suitable 

 electrolyte, usually bicarbonate of soda, which allows the passage 



