INSTRUMENTS, OBSERVATIONAL PROCEDURE, AND CONSTANTS 



13 



than ten particles before some of them evaporate is very 

 difficult, and the piston setting should always be such as 

 to keep the number deposited at any one expansion at 

 some value smaller than ten per square millimeter. 



On cruise VII the practice was adhered to of taking 

 ten successive determinations of nuclei content as one 

 set of observations. Such a set required between ten and 

 fifteen minutes for completion. The sets were made to 

 coincide with the mid-point in time of the conductivity 

 and ion-content measurements, thus making the various 

 types of data simultaneous. Early in the cruise it was 

 found that the nuclei were not numerous over the ocean 

 and the observing program was modified by counting the 

 particles falling on four squares instead of only on one 

 square. The constants used in connection with various 

 settings on the piston tube were therefore only one- 

 fourth the values given in the preceding table. 



A suitable form (designated number 157) was pro- 

 vided for recording the nuclei counts. On the form space 

 was provided for weather notes relating to wind direc- 

 tion and velocity, visibility, presence of fog, mist, or 

 haze, and other items which might be pertinent to the 

 nuclei concentration in the atmosphere. 



Penetrating Radiation Apparatus PRl and Kolhorster 

 5503. --The measurement of the penetrating radiation 

 consists essentially in determining the rate of formation 

 of ions in the air of a closed chamber. This is much the 

 same in principle as measuring the number of ions with 

 an ion counter. All the ions of one sign are swept to an 

 electrode (or central system), near the center of the 

 ionization chamber, by a suitable electric field. Thus, 

 in apparatus of the type of PRl, the central system (in- 

 cluding the central electrode, the fiber, and other con- 

 nected parts) gradually acquires charge, and in an in- 

 strument of the KolhSrster type it loses a charge. The 

 time (r) required for the central system to suffer a 

 change of potential (6V), indicated by a change in fiber 

 position {$) is noted. Then the charge in electrostatic 

 units acquired per second is Ci5Vt -1/300, where C is 

 the capacitance of the entire central system, 6V is ex- 

 pressed in volts and t in seconds (the symbols here 

 used are the same as on Department form 104). The 

 charge acquired per second divided by the charge (e) of 

 a simple ion gives the number of ions collected by the 

 central electrode per second and this in turn divided by 

 the volume (U) of the ionization chamber yields the 

 number of pairs of ions (R) formed per second in one 

 cubic centimeter of the air contained in the chamber . 

 Thus 



R = C(5VT-V300Ue 



Chief Features of Penetrating Radiation Apparatus 

 1.--A cylindrical chamber approximately 29 cm in di- 

 ameter and 34 cm high, was made from copper sheet ap- 

 proximately 0.1 cm in thickness. It was thoroughly 

 cleaned and filled with dry filtered air. Mounted on suit- 

 able supports, it was insulated from earth and, being con- 

 nected with one of the plates of a unifQar electrometer, 

 was maintained at a potential of about 100 volts (fig. 4). 

 Concentric within the lower two-thirds of the cylinder 

 was a rod, or central electrode, which was connected with 

 the electrometer fiber and with the inner element of a 

 small condenser, the latter being mounted on the side of 

 the electrometer cap. The other element of this condens- 

 er, the outer cylinder, was insulated from earth and 

 connected with that electrometer plate which was not 



connected with the ionization chamber. The potential on 

 this outer cylinder therefore was equal in magnitude but 

 opposite in sign to that on the ionization chamber, pro- 

 vided all insulation on the lines connecting with the bat- 

 teries was adequate. The capacitance of the so-called 

 balancing condenser could be varied by small amounts 

 by means of the adjusting screw at the end of the outer 

 cylinder, but for greater changes it was necessary to re- 

 move the outer cylinder and screw the inner cylinder in 

 or out as required. The balancing condenser was correct- 

 ly set at the laboratory in Washington and instructions 

 were to avoid changing it, if possible, as the adjustment 

 would be a tedious operation. The purpose of the con- 

 denser was to annul inductive effects that, without it, 

 would occur from very small variations in the battery 

 potential and would falsify the measurements. 



All the prime insulators of the central system were 

 surrounded by earthed guard-rings and, since the poten- 

 tial of that system differed very little from zero, the 

 loss of charge over these insulators could not be large. 

 Furthermore, the observations were so arranged that 

 the average potential of the central system was zero 

 (e.g., an initial potential of +1 and a final potential of -1) 

 so that the loss and gain would nearly balance. 



Two drying tubes were attached to the ionization 

 chamber. The drying material was metallic sodium which 

 lasted for long periods provided air was not admitted 

 to the chamber. During use, the sodium would change to 

 sodium hydroxide on absorbing moisture, and the latter 

 was itself a fairly good drying agent and could be left 

 until droplets of the liquid solution appeared in the tubes. 



The constants of PRl were: capacitance, C, 12.8 

 cm; volume, U, 21,600 cc. These, together with the 

 value for the charge of a simple ion (e = 4.77 x 10-10), 

 were used in the equation given above and, when com- 

 bined, gave as the working formula for this instrument 



R =4140 5Vt-1 



Special Notes Regarding Care and Adjustment of 

 PRl. -- When a change of dryer became necessary, the 

 change was made in such a way as to minimize the 

 chance of an interchange of air between the chamber 

 and the outside. The procedure was that one of the spare 

 drying tubes would be filled with fresh sodium before the 

 change and so would be ready to screw into place as soon 

 as the exhausted tube was removed. 



The insulators on the battery lines were those which 

 required most attention. To be satisfactory the insulation 

 resistance on each battery line needed to be about 109 

 ohms. Tests to determine whether or not this require- 

 ment was fulfilled were made in accordance with instruc- 

 tions. Other insulators which were carefully watched for 

 leak were those on the ionization chamber and the out- 

 side insulator on the balancing condenser. 



Adjustments of the electrometer were necessary at 

 times, both to obtain the desired sensitivity and to obtain 

 about the same sensitivity on either side of zero. If the 

 latter conditions were not approximately maintained, the 

 loss and gain of charge by leakage across the prime in- 

 sulators did not balance. Whenever adjustments of the 

 electrometer were made, it was necessary that both the 

 ionization chamber and the balancing condenser be dis- 

 connected from the electrometer plates and earthed. 

 After completion of the adjustments, the zero-positions 

 were noted for both settings of the battery reversing 

 switch. With these zero-positions in mind, the ionization 



