1849 became professor of music at the college. His great interest in experimental science led to his undertaking the teaching of natural philosophy, and during the tenure of his double office the idea of his type-printing telegraph occurred to him. Although (Sir) Charles Wheatstone [q. v.] had exhibited a type-printer at the Royal Polytechnic Institution, London, in 1841, the first instrument available for practical use was that invented by House, of Vermont, and adopted by the American Telegraph Company in 1847. In it the motion of the wheel carrying the type at the receiving station was produced step by step, by the teeth of a wheel at the transmitting end making and breaking the electrical circuit as it was rotated. Hughes proposed to produce these synchronous rotations mechanically, and only to use the electric current once for each letter printed.
He resigned his position at Bardstown, and spent two years working out the details of his instrument, which he completed and patented in 1856. Next year it was adopted by the American Telegraph Company, and many of its features are present in the Phelps instruments now used by them.
In 1857 Hughes brought the instrument to this country, and, on its not meeting with the reception he expected, proceeded to France, where it was purchased by the government in 1860 and installed on their lines. During the next ten years it was adopted by most of the continental governments, and its inventor was the recipient of many decorations and honours. In 1872, while resident in Paris, he was elected a foreign member of the newly founded Society of Telegraph Engineers, now the Institution of Electrical Engineers. In 1877 he settled in London, and devoted much of his time to experimental electrical work, with apparatus constructed by himself.
The telephone, invented by Reiss in 1861, had been rendered a practical instrument by Bell in 1876, but his transmitter was still unsatisfactory, even after the introduction of the carbon button into it in 1877. Further improvement was rendered possible by the invention of the 'microphone' in 1878, almost simultaneously by Lüdtge ('universal telephone,' German patent, 12 Jan. 1878), and by Hughes (Proc. Royal Soc. London, 8 May 1878). It owes its action, as the latter explained, to the great variation of electrical resistance of a loose contact between two conductors, on the slightest relative motion of the two parts.
In April 1878 D'Arsonval, in a communication to the Académie des Sciences (Comptes Rendus, lxxxvi. 832), called attention to the telephone as a sensitive detector of varying electric currents, and in May 1879 Hughes exhibited to the Royal Society of London (Proc. Royal Soc. xxix. 56) a new 'induction balance,' in which a telephone replaced the galvanometer and current rectifier of Felici (Ann. de Chim. et de Phys. xxxiv. 65, 68, 1852), and with it repeated and extended the results obtained by Dove with his original balance (Ann. der Physik, xlix. 77, 1840).
In 1880 he was elected a fellow of the Royal Society, and in 1885 received the society's gold medal 'for experimental research in electricity and magnetism, and for the invention of the microphone and induction balance.' He had ceased to be a foreign and become an ordinary member of the Society of Telegraph Engineers in 1879, and after being successively a member of the council (1880) and vice-president (1882), he was in 1886 elected president of the society. In his inaugural address he gave an account of his experiments on 'the self-induction of an electric current,' &c. (Journal Tel. Eng. xv. 6), and succeeded in arousing general interest in the laws of distribution of alternating electric currents in conductors, which had been investigated mathematically by Heaviside and others.
During the interval 1879-86 Hughes appears from his letters to have convinced himself by experiment of the existence of electric waves in the air surrounding an electric spark, and to have discovered the efficacy of a microphone contact (coherer) in series with a telephone or galvanometer and a voltaic cell, as a detector of them. Unfortunately these early experiments on aerial telegraphy were not made public, and it was left for Hertz to demonstrate the existence of electric waves in 1887, for Branly to re-invent the coherer as a detector in 1891, and for Marconi to combine the two into a system of wireless telegraphy in 1896.
He continued for the rest of his life to take an interest in electrical matters, and occasionally took part in the discussion of papers read before the Institution of Electrical Engineers. In 1889 he was elected a manager, and in 1891 vice-president, of the Royal Institution. In 1898 the Society of Arts conferred the Albert medal on him for 'his numerous inventions, especially the printing telegraph and the microphone.'
About this time he began to be troubled with paralysis, and died at 40 Langham Street, W., on 22 Jan. 1900, after an attack of influenza. He was interred at Highgate cemetery. Leaving no issue, he bequeathed between 300,000l. and 400,000l. to four