Soapstone, biscuit porcelain and a variety of insulating sub stances have also been tried as bases. On the other hand the proc ess known as fuming and already described as suitable for the formation of crystals, generally gives an adherent layer even upon polished glass. The presence of oil improves the contact. In this way experiments may be made on the light sensitivity of a single crystal.
Cylindrical bridges made by placing alternate discs of brass and mica upon an insulating rod are useful for certain purposes. The discs are clamped by tight ening a nut on the rod and the brass plates are alternately con nected together into two groups to which terminals are attached. The whole is heated and selenium melted over the outer edges of the discs, the usual precaution being taken to convert it into the grey light-sensitive variety. Two con ducting plates immersed in various liquids, so as to constitute a voltaic battery, one plate being coated with grey selenium and exposed to light, have also been found to be effective as a light sensitive device. Bridges made with pairs of dissimilar metals separated by thin mica and coated with selenium give an electric current on illumination. Whether a cell will have a high or low electrical resistance when finished depends upon the temperature and time taken in its preparation. It is not uncommon to find bridges designed to be operated for long periods on 200 volts.
It was formerly held that impurities were necessary but Marc has definitely proved that view to be incorrect. He showed that the presence of traces of silver, etc., affect the behaviour of the cells but proved that the fundamental action of light upon pure selenium is indeed a fact. He supported these ideas by various ex periments, but failed to devise any really decisive proof of the cor rectness of his conclusions. The more modern view is, of course, based upon the existence of electrons and seems to adequately ex plain many of the experimental results. According to the electron theory, selenium becomes ionised by light, and the time-lag is accounted for by supposing the recombination of the ions to take place more slowly than in the case of a gas. That is of course
a difficulty, but the electron theory also requires that the resis tance of selenium bridges should vary as the square root of the intensity of light falling upon them, and this is found to be the case. On the other hand the sensitive crystals are surprisingly opaque to light and it appears that light waves are only able to penetrate them to a depth of .0014 cm. In addition there is the fact that surface conditions can vary widely without affecting the sensitivity, and it therefore seems that if electrons are produced or liberated by the light, the process must be one occurring within the crystal itself. It may be generally stated that selenium does not show any selective action towards the colour of the light impinging upon it : the chief factor appears to be the energy con veyed by the beam. In most sources of light the greatest energy is carried by the longer wave lengths and it was therefore formerly thought that selenium is most sensitive to red light. As a matter of fact, for coloured lights with equal energy, selenium is usually somewhat more sensitive to blue light. It also responds to ultra violet light and X-rays as well as to the gamma rays from radium. Experiments upon the sensitivity of selenium to light of various wave lengths have shown that the temperature treatment of the bridge appreciably influences the result.
Prof. F. C. Brown has shown, that when using a single crystal of selenium 1 cm. long, the maximum change in resistance across its base occurred when the extreme tip only was illuminated. The crystals may be bent or flattened but still even after heavy pres sure retain their sensitivity to light. A block of grey or metallic selenium is sensitive to pressure, and filing or scraping also definitely increases its resistance. If the light falls upon a block of selenium having wire gauze electrodes, the change of resistance is greater in the direction of the current than at right angles to it.