Before the transistor was invented and made available for the industry, vacuum tubes were used for signal processing of any kind(radios, transmitters, amplifiers etc).
The summary of tube history will be short, I promise. Only the most important facts:
1904 the vacuum diode was invented by John Ambrose Flemming, it was used to rectify any kind of alternate current. 1906 consequently the triode was invented by Lee de Forest. For the first time in history it was possible to amplify electric signals. The other types of tubes(tetrode, pentode, hexode, heptode, octode, nonode) are triodes with more grids. They all had been invented till 1935.
I almost forgot the beam power tetrodes and pentodes. The Americans invented them to bypass european patent rights. Last but not least, the indicator tubes, like EM4, EM34, EM84(just to mention a few of them). They look really awesome, when in use.
If someone wants to know more, just read the wikipedia article about it(I cant link my books to the page :D)
Before led's, VFD's and early LCD's were available, nixie tubes were used to display numbers and characters. They work like a gas discharge tube with a plate anode in the back or around them,
depending on the type of nixie and a mesh anode in front or above the characters so you can see them. The characters themselves are the cathode. The usually have a working voltage around 150V and
draw 2mA, when a segment is switched on. Nixies are driven by special driver IC's like the 74141 or the russian pendant. You can also use a simple BC546 or a ULN2803A, but the
ULN2803A has two switches less than needed. So you might youse to of the in a nixie clock and you don't have to buy special nixie driver IC's. There is also a HV version of the 74HC595 that could
Nixies are still available today, although fore some types, especially old German ones, really high prices are paid. In that case, go for some Russian nixies. They cost less and look beautiful too. You can also be lucky and get some German nixies at a very good prices on eBay.
The nixies leaning at the Monster Rehab can (my favorite energy drink) are Z574M German made. They display the numbers from 0-9.
The one's with the black socket are Russian IN12B. The B stands for the decimal point. They display the numbers from 0-9.
The lonely nixie Tube is a Russian IN15B. It displays A, V, W, Hz, the Ohm symbol, F.
Those four tiny nixies in the white box are German ZM1325. They display the numbers from 0-9.
I recentyl got two GR22-214 nixie tube with a secondary anode for stability. The are very old and feature a Europa 4-pin socket. The also come with a mesh anode. Therefore you can see them glow very good. I put a short video on youtube: GR22-214