July 12, 1907] 



SCIENCE 



53 



tion falls very considerably. It will also be 

 noticed that the relative minimum drop on 

 March 10 is not nearly as large as on March 

 9. The dotted portions of the curve repre- 

 sent the times when the electroscope was re- 

 charged. That all parts of the scale were 

 equally sensitive was shown afterwards when 

 a lead screen was placed around the electro- 

 scope. The rate of fall of the gold leaf was 

 then practically constant for the portion of 

 path used. During the day there was a great 

 deal of vibration due to travel on the cobble- 

 stone street nest the building, so that the 

 error of reading was larger. 



Fig. 2 represents the ionization for several 

 days in February and March of the present 

 year, (a) is the average of readings for four 

 days and (&) for fifteen days. These and 

 other curves show maxima for February and 

 March at about 9 a.m. and 10 p.m., and minima 

 at about 7 a.m. and 6 p.m. Very few observa- 

 tions have as yet been made on the 7 a.m. 

 and 9 A.M. periods. The most conspicuous is 

 the minimum at 6 p.m. This occurs with con- 

 siderable regularity and is very marked, the 

 ionization often falling thirty or forty per 

 cent. No corresponding change of tempera- 

 ture or barometric pressure was noticed. It 

 will be noticed in the following table, how- 

 ever, that it never drops below the value of 

 the ionization when the penetrating radiation 

 is cut off. It has been found that sudden 

 changes of temperature produce air currents 

 and set the gold leaf in motion, but it hardly 

 seems likely that this minimum is to be 

 explained in this way. Still it seems very 

 remarkable that the penetrating radiation 

 should have such a marked drop and the 

 problem as to whether it is a temperature 

 effect is to be taken up. 



The electroscope was then screened with 

 lead plates from 4 to 5 cm. thick. A window 

 was necessary to make the readings, how- 

 ever, so that the radiation was not all 

 screened out. The rate of leak was made 

 much more constant. The marked minimum 

 at 6 P.M. was usually not noticeable. 



The following table gives the average ioni- 

 zation for several days. Headings were usu- 

 ally taken from 10 a.m. till 6 p.m. The period 



of minimum is not included in the column 

 marked average ionization. The ionization 

 during the early part of the afternoon waa 

 found fairly constant. 



In conclusion the writer wishes to express 

 his thanks to Professor Ames for his many 

 kindnesses and to Professors Rutherford and 

 Dike for their suggestions. 



W. W. Strong 



LABORATORY APPARATUS FOR MEASUREMENT OF 

 THE FORCE ON A CURRENT-CARRYING CON- 

 DUCTOR LYING IN A MAGNETIC FIELD 



The method used by Ampere in his investi- 

 gations of the effect of a magnetic field on a 

 current-carrying conductor was to arrange the 

 conductor so that the forces acting on one part 

 of the circuit just balanced those acting on 

 another part. From observations thus made 

 and without the direct measurement of any 

 forces in force units, Ampere established his 

 propositions with regard to the mutual action 

 of current-carrying conductors. 



From these propositions is derived the ex- 

 pression for the force acting on a straight 

 conductor lying in a uniform magnetic field. 

 This is a very special application, but it is 



