Thus, if we received 4 different symbols occurring with equal frequency, this would represent 2 bits of information. It is likely that Hartley was aware of the earlier work of Nyquist and that he assumed implicitly, as Nyquist did explicitly, that all symbol sequences were of the same length or size. Symbols need not be equi-probable for Hartley's formula to be correct if symbols are of equal length. Hartley was aware of a relationship between the amount of energy in an information system and the amount of information that could be transmitted. Applying energy to an information transmitting system increases the ease with which the recipient receives or hears the transmitted signal.
Energy serves as a component of the transmission process.
Increasing the signal to noise ratio increases the probability that the information will be received correctly. During World War II, Claude Shannon developed a model of the communication process using the earlier work of Nyquist and Hartley. Published in 1947, The Mathematical Theory of Communication became the founding document for much of the future work in information theory. Shannon thought that the fundamental problem of communication is that of reproducing at one point either exactly or approximately a message selected at another point. These semantic aspects of communication are irrelevant to the engineering problem.