Nikola Tesla: Colorado Springs Notes, 1899-1900 Page 424

carry out measurements on individual bottles to determine what kind of tolerance they had. Since he only measured complete banks, i.e. rather large capacities, and mentions “readings with 7 cells battery”, he probably made the comparison in terms of stored charges.

October 28

After several days gathering data and making further measurements of inductance and capacitance, he finally proceeds to the calculation of the unknown capacity of the sphere, using the readings of October 23rd and 26th. Consistent with his general principle, he checked the measured values by (usually approximate) calculations.

He calculates the capacitance of the vertical wire by the formula for an isolated ellipsoid of high eccentricity. It is not known whether Tesla first had the idea of using this formula, but it was used later for a similar purpose⁽⁵⁸⁾. All things considered, the agreement between the calculated and measured values is very good.

Tesla then calculated the ratio of the capacities in the lowest and highest positions. In the lowest position the sphere makes little difference to the total capacity. In the highest position it increases the capacity in the coil circuit by 18.7 cm. This is less than the theoretical value for an isolated sphere of 18" diameter, which does not agree with some of his earlier measurements (see the results of October 21st for a 30" ball). However, if a comparison is to be made, it must be noted that the new results are probably better because the apparatus had been modified and the parameters checked.

October 29

His remark about eddy currents in the sphere is interesting. To prevent their formation he slit the tinfoil with a knife. Did he assume that in the vicinity of the coil the sphere would behave like a short-circuited turn? It is readily shown that if this effect is pronounced (and not taken into account) the measured capacity of the sphere will be too low. This might be an explanation for the reduction of the effective capacity of the sphere in the lowest position (see the calculation of October 28th for an 18" sphere in the lowest position).

November 1-2

The new extra coil was larger in diameter but shorter than the previous one (see August 23rd). The formula for an infinite coil introduces a rather large error, but with the correction referred to in the commentary to 26 October the agreement with the experimental results is good. The correction terms are significant because the ratio D/l is even greater than unity, and the correction is more than 30%. The corrected value is 0.0198 H, 2 - 3% less than the measured values.

November 3

A repeated measurement of the capacity of the vertical wire and the 30" sphere by the method of October 29th but with a new coil L (see October 31st). A new feature is Tesla’s attempt to eliminate the wires of the spark gap, estimating the excitation solely from the streamers.