How did Jocelyn Bell Burnell discover the periodicity of CP 1919?
Supplemental to @RoryAlsop's answer refer to Astrobytes' The First Pulsar which discusses the Nature paper (A. Hewish, S. J. Bell, J. Pilkington, P. Scott, and R. Collins 1968) Observation of a Rapidly Pulsating Radio Source and shows Figure 2 from the paper.
For more on the radio telescope itself see What exactly is interplanetary scintillation; what was the Interplanetary Scintillation Array looking for? Did it successfully observe any?
The original is still paywalled but Nature has reprinted a bit of it here. It can also be downloaded from Researchgate.
Here is a detail of the image from the Astrobytes post and the caption there.
Figure 2: Data from the Interplanetary Scintillation Array (IPS) radio interferometer at Mullard Radio Astronomy Observatory in 1967. The periodic increase in flux is on the scale of seconds and show the very first measurements of a pulsar.
Here is the actual caption from the original Nature paper. What's interesting to see is that in addition to using the speed of the chart recorder as a timebase, an external pulser was used for reference. Also notable is the delay between pulses at one frequency and another, which the authors discuss and attribute to dispersion in the interstellar medium. They see about 0.2 second delay between 80.5 and 81.5 MHz and cite a -4.9+/- 0.0-5 MHz/sec frequency sweep. Using 0.2 electrons per cm^3 the mention an upper limit for the distance as 65 parsecs.
Fig. 1. a, A record of the pulsating radio source in strong signal conditions (receiver time constant 0·1 s). Full scale deflection corresponds to 20 x 10-26 W m-2 Hz-1. b, Upper trace: records obtained with additional paths 240m and 450 ml in one side of the interferometer. Lower trace· normal interferometer records. The pulses are small for l = 240 m because they occurred near a null in the interference pattern. (This modifies the phase but not the amplitude of the oscillatory response on the upper trace.) c, Simulated pulses obtained using a signal generator. d, Simultaneous reception of pulses using identical receivers tuned to different frequencies. Pulses at the lower frequency are delayed by about 0.2 seconds.