dissipation factor or loss tangent. The dissipation factor (D) or loss tangent 

 (tan 5) are related to the loss current (I,), to the charging current (I^), and to 

 the resistance and capacitance, as follows: ^^ 



D = tan 5 = 



I^ coRC 



As a coating deteriorates it might be expected that the loss current 

 and the dissipation factor increase. This would nnean that the resistance must 

 decrease more rapidly than the capacitance increases. For a simple loss of 

 thickness not accompanied by other changes, no change in dissipation factor 

 would be expected. 



The resistance, capacitance, and dissipation factor at a frequency of 

 1 kHz can readily be determined with equipment such as that described above. 

 Direct current resistance cannot be determined by a simple resistance measure- 

 ment because the direct current potential necessary would cause considerable 

 polarization. The method which has been used by others and which was 

 slightly modified for the present work considers the panel, the coating system, 

 the seawater, and the calomel electrode as a cell, or source of electromotive 

 force. The resistance of the coating system is effectively the internal resistance 

 of this cell. From the open cell potential (E^) and the shunted cell potential 

 (Ej) obtained when the cell is shunted with a resistance (Rj), as shown in 

 Figure 24, the internal resistance (R,) is calculated: 



/e„ - eA 



In arriving at the above equation, it is assumed that there is no change 

 in the basic electromotive force of the cell (E) as current is drawn, and that 

 the potential across the internal resistance (E|) is in fact the difference between 

 the open-cell and closed-cell potentials. 



Ei = E - E3 = E„ - E3 



In practice there will be a drop in the potential of the cell (E) due to polarization, 

 and it will be smaller than E^. Thus, Ej, and therefore Rj, will be considerably 

 less than the values calculated. This may be the chief reason why reported DC 

 resistances have been much higher than reported AC resistances. 



33 



